Convolution#Measures

The convolution of $f$ and $g$ is written $f\; *\; g$, denoting the operator with the symbol $*$.{{efn-ua

| The symbol {{unichar|2217|asterisk operator}} is different than {{unichar|2A|asterisk}}, which is often used to denote complex conjugation. See {{slink|Asterisk|Mathematical typography}}.

}} It is defined as the integral of the product of the two functions after one is reflected about the y-axis and shifted. As such, it is a particular kind of integral transform:

:$(f\; *\; g)(t)\; :=\; \backslash int\_\{-\backslash infty\}^\backslash infty\; f(\backslash tau)\; g(t\; -\; \backslash tau)\; \backslash ,\; d\backslash tau.$

An equivalent definition is (see commutativity):

:$(f\; *\; g)(t)\; :=\; \backslash int\_\{-\backslash infty\}^\backslash infty\; f(t\; -\; \backslash tau)\; g(\backslash tau)\backslash ,\; d\backslash tau.$

While the symbol $t$ is used above, it need not represent the time domain. At each $t$, the convolution formula can be described as the area under the function $f(\backslash tau)$ weighted by the function $g(-\backslash tau)$ shifted by the amount $t$. As $t$ changes, the weighting function $g(t-\backslash tau)$ emphasizes different parts of the input function $f(\backslash tau)$; If $t$ is a positive value, then $g(t-\backslash tau)$ is equal to $g(-\backslash tau)$ that slides or is shifted along the $\backslash tau$-axis toward the right (toward $+\backslash infty$) by the amount of $t$, while if $t$ is a negative value, then $g(t-\backslash tau)$ is equal to $g(-\backslash tau)$ that slides or is shifted toward the left (toward $-\backslash infty$) by the amount of $|t|$.

For functions $f$, $g$ supported on only $[0,\backslash infty)$ (i.e., zero for negative arguments), the integration limits can be truncated, resulting in:

:$(f\; *\; g)(t)\; =\; \backslash int\_\{0\}^\{t\}\; f(\backslash tau)\; g(t\; -\; \backslash tau)\backslash ,\; d\backslash tau\; \backslash quad\; \backslash \; \backslash text\{for\; \}\; f,\; g\; :\; [0,\; \backslash infty)\; \backslash to\; \backslash mathbb\{R\}.$

For the multi-dimensional formulation of convolution, see domain of definition (below).

A common engineering notational convention is:

{{cite book|last1=Smith|first1=Stephen W|title=The Scientist and Engineer's Guide to Digital Signal Processing|date=1997|publisher=California Technical Publishing|isbn=0-9660176-3-3|edition=1|chapter-url=https://dspguide.com/ch13/2.htm|access-date=22 April 2016|chapter=13.Convolution}}

:$f(t)\; *\; g(t)\; \backslash mathrel\{:=\}\; \backslash underbrace\{\backslash int\_\{-\backslash infty\}^\backslash infty\; f(\backslash tau)\; g(t\; -\; \backslash tau)\backslash ,\; d\backslash tau\}\_\{(f\; *\; g\; )(t)\},$

which has to be interpreted carefully to avoid confusion. For instance, $f(t)\; *\; g(t-t\_0)$ is equivalent to $(f*g)(t-t\_0)$, but $f(t-t\_0)\; *\; g(t-t\_0)$ is in fact equivalent to $(f\; *\; g)(t-2t\_0)$.{{cite book|last1=Irwin|first1=J. David|author-link=J. David Irwin|title=The Industrial Electronics Handbook|date=1997|publisher=CRC Press|location=Boca Raton, FL|isbn=0-8493-8343-9|page=75|edition=1|chapter=4.3}}

Given two functions $f(t)$ and $g(t)$ with bilateral Laplace transforms (two-sided Laplace transform)

:$F(s)\; =\; \backslash int\_\{-\backslash infty\}^\backslash infty\; e^\{-su\}\; \backslash \; f(u)\; \backslash \; \backslash text\{d\}u$

and

:$G(s)\; =\; \backslash int\_\{-\backslash infty\}^\backslash infty\; e^\{-sv\}\; \backslash \; g(v)\; \backslash \; \backslash text\{d\}v$

respectively, the convolution operation $(f\; *\; g)(t)$ can be defined as the inverse Laplace transform of the product of $F(s)$ and $G(s)$.{{cite web |last1=Differential Equations (Spring 2010) |first1=MIT 18.03 |title=Lecture 21: Convolution Formula |url=https://ocw.mit.edu/courses/mathematics/18-03-differential-equations-spring-2010/video-lectures/lecture-21-convolution-formula/ |website=MIT Open Courseware |publisher=MIT |access-date=22 December 2021}}{{cite web |title=18.03SC Differential Equations Fall 2011 |url=https://ocw.mit.edu/courses/mathematics/18-03sc-differential-equations-fall-2011/unit-iii-fourier-series-and-laplace-transform/transfer-system-and-weight-functions-greens-formula/MIT18_03SCF11_s30_5text.pdf |archive-url=https://web.archive.org/web/20150906102242/https://ocw.mit.edu/courses/mathematics/18-03sc-differential-equations-fall-2011/unit-iii-fourier-series-and-laplace-transform/transfer-system-and-weight-functions-greens-formula/MIT18_03SCF11_s30_5text.pdf |archive-date=2015-09-06 |url-status=live |website=Green's Formula, Laplace Transform of Convolution}} More precisely,

:$$

\begin{align}

F(s) \cdot G(s) &= \int_{-\infty}^\infty e^{-su} \ f(u) \ \text{d}u \cdot \int_{-\infty}^\infty e^{-sv} \ g(v) \ \text{d}v \\

&= \int_{-\infty}^\infty \int_{-\infty}^\infty e^{-s(u + v)} \ f(u) \ g(v) \ \text{d}u \ \text{d}v

\end{align}

Let $t\; =\; u\; +\; v$, then

:$$

\begin{align}

F(s) \cdot G(s) &= \int_{-\infty}^\infty \int_{-\infty}^\infty e^{-st} \ f(u) \ g(t - u) \ \text{d}u \ \text{d}t \\

&= \int_{-\infty}^\infty e^{-st} \underbrace{\int_{-\infty}^\infty f(u) \ g(t - u) \ \text{d}u}_{(f * g)(t)} \ \text{d}t \\

&= \int_{-\infty}^\infty e^{-st} (f * g)(t) \ \text{d}t.

\end{align}

Note that $F(s)\; \backslash cdot\; G(s)$ is the bilateral Laplace transform of $(f\; *\; g)(t)$. A similar derivation can be done using the unilateral Laplace transform (one-sided Laplace transform).

The convolution operation also describes the output (in terms of the input) of an important class of operations known as linear time-invariant (LTI). See LTI system theory for a derivation of convolution as the result of LTI constraints. In terms of the Fourier transforms of the input and output of an LTI operation, no new frequency components are created. The existing ones are only modified (amplitude and/or phase). In other words, the output transform is the pointwise product of the input transform with a third transform (known as a transfer function). See Convolution theorem for a derivation of that property of convolution. Conversely, convolution can be derived as the inverse Fourier transform of the pointwise product of two Fourier transforms.

One of the earliest uses of the convolution integral appeared in D'Alembert's derivation of Taylor's theorem in Recherches sur différents points importants du système du monde, published in 1754.Dominguez-Torres, p 2

Also, an expression of the type:

:$\backslash int\; f(u)\backslash cdot\; g(x\; -\; u)\; \backslash ,\; du$

is used by Sylvestre François Lacroix on page 505 of his book entitled Treatise on differences and series, which is the last of 3 volumes of the encyclopedic series: {{Lang|fr|Traité du calcul différentiel et du calcul intégral}}, Chez Courcier, Paris, 1797–1800.Dominguez-Torres, p 4 Soon thereafter, convolution operations appear in the works of Pierre Simon Laplace, Jean-Baptiste Joseph Fourier, Siméon Denis Poisson, and others. The term itself did not come into wide use until the 1950s or 1960s. Prior to that it was sometimes known as Faltung (which means folding in German), composition product, superposition integral, and Carson's integral.

{{Citation

| chapter = Early work on imaging theory in radio astronomy

| author = R. N. Bracewell

| editor = W. T. Sullivan

| title = The Early Years of Radio Astronomy: Reflections Fifty Years After Jansky's Discovery

| publisher = Cambridge University Press

| year = 2005

| isbn = 978-0-521-61602-7

| page = 172

| chapter-url = https://books.google.com/books?id=v2SqL0zCrwcC&pg=PA172

}} Yet it appears as early as 1903, though the definition is rather unfamiliar in older uses.

{{Citation

| title = The algebra of invariants

| author = John Hilton Grace and Alfred Young

| publisher = Cambridge University Press

| year = 1903

| page = 40

| url = https://books.google.com/books?id=NIe4AAAAIAAJ&pg=PA40

}}

{{Citation

| title = Algebraic invariants

| author = Leonard Eugene Dickson

| publisher = J. Wiley

| year = 1914

| page = 85

| isbn = 978-1-4297-0042-9

| url = https://books.google.com/books?id=LRGoAAAAIAAJ&pg=PA85

}}

The operation:

:

is a particular case of composition products considered by the Italian mathematician Vito Volterra in 1913.

According to

[Lothar von Wolfersdorf (2000), "Einige Klassen quadratischer Integralgleichungen",

Sitzungsberichte der Sächsischen Akademie der Wissenschaften zu Leipzig,

Mathematisch-naturwissenschaftliche Klasse, volume 128, number 2, 6–7], the source is Volterra, Vito (1913),

"Leçons sur les fonctions de linges". Gauthier-Villars, Paris 1913.

{{Main article|Circular convolution}}

When a function $g\_T$ is periodic, with period $T$, then for functions, $f$, such that $f\; *\; g\_T$ exists, the convolution is also periodic and identical to:

:$(f\; *\; g\_T)(t)\; \backslash equiv\; \backslash int\_\{t\_0\}^\{t\_0+T\}\; \backslash left[\backslash sum\_\{k=-\backslash infty\}^\backslash infty\; f(\backslash tau\; +\; kT)\backslash right]\; g\_T(t\; -\; \backslash tau)\backslash ,\; d\backslash tau,$

where $t\_0$ is an arbitrary choice. The summation is called a periodic summation of the function $f$.

When $g\_T$ is a periodic summation of another function, $g$, then $f*g\_T$ is known as a circular or cyclic convolution of $f$ and $g$.

And if the periodic summation above is replaced by $f\_T$, the operation is called a periodic convolution of $f\_T$ and $g\_T$.

File:2D Convolution Animation.gif

For complex-valued functions $f$ and $g$ defined on the set $\backslash Z$ of integers, the discrete convolution of $f$ and $g$ is given by:{{harvnb|Damelin|Miller|2011|p=219}}

:$(f\; *\; g)[n]\; =\; \backslash sum\_\{m=-\backslash infty\}^\backslash infty\; f[m]\; g[n\; -\; m],$

or equivalently (see commutativity) by:

:$(f\; *\; g)[n]\; =\; \backslash sum\_\{m=-\backslash infty\}^\backslash infty\; f[n-m]\; g[m].$

The convolution of two finite sequences is defined by extending the sequences to finitely supported functions on the set of integers. When the sequences are the coefficients of two polynomials, then the coefficients of the ordinary product of the two polynomials are the convolution of the original two sequences. This is known as the Cauchy product of the coefficients of the sequences.

Thus when {{mvar|g}} has finite support in the set $\backslash \{-M,-M+1,\backslash dots,M-1,M\backslash \}$ (representing, for instance, a finite impulse response), a finite summation may be used:{{cite book |last1=Press |first1=William H. |last2=Flannery |first2=Brian P. |last3=Teukolsky |first3=Saul A. |last4=Vetterling |first4=William T. |title=Numerical Recipes in Pascal |year=1989 |publisher=Cambridge University Press |isbn=0-521-37516-9 |page=[https://archive.org/details/numericalrecipes0000unse/page/450 450] |url=https://archive.org/details/numericalrecipes0000unse/page/450}}

:$(f\; *\; g)[n]=\backslash sum\_\{m=-M\}^M\; f[n-m]g[m].$

When a function $g\_\{\_N\}$ is periodic, with period $N,$ then for functions, $f,$ such that $f*g\_\{\_N\}$ exists, the convolution is also periodic and identical to:

:$(f\; *\; g\_\{\_N\})[n]\; \backslash equiv\; \backslash sum\_\{m=0\}^\{N-1\}\; \backslash left(\backslash sum\_\{k=-\backslash infty\}^\backslash infty\; \{f\}[m\; +\; kN]\backslash right)\; g\_\{\_N\}[n\; -\; m].$

The summation on $k$ is called a periodic summation of the function $f.$

If $g\_\{\_N\}$ is a periodic summation of another function, $g,$ then $f*g\_\{\_N\}$ is known as a circular convolution of $f$ and $g.$

When the non-zero durations of both $f$ and $g$ are limited to the interval $[0,N-1],$  $f*g\_\{\_N\}$ reduces to these common forms:

{{Equation box 1|title=

|indent=: |cellpadding= 0 |border= 0 |background colour=white

|equation={{NumBlk2|

|$\backslash begin\{align\}$

\left(f * g_N\right)[n] &= \sum_{m=0}^{N-1} f[m]g_N[n - m] \\

&= \sum_{m=0}^n f[m]g[n - m] + \sum_{m=n+1}^{N-1} f[m]g[N + n - m] \\[2pt]

&= \sum_{m=0}^{N-1} f[m]g[(n - m)_\bmod{N}] \\[2pt]

&\triangleq \left(f *_N g\right)[n]

\end{align}        

|Eq.1}}}}

The notation $f\; *\_N\; g$ for cyclic convolution denotes convolution over the cyclic group of modular arithmetic.

Circular convolution arises most often in the context of fast convolution with a fast Fourier transform (FFT) algorithm.

In many situations, discrete convolutions can be converted to circular convolutions so that fast transforms with a convolution property can be used to implement the computation. For example, convolution of digit sequences is the kernel operation in multiplication of multi-digit numbers, which can therefore be efficiently implemented with transform techniques ({{harvnb|Knuth|1997|loc=§4.3.3.C}}; {{harvnb|von zur Gathen|Gerhard|2003|loc=§8.2}}).

{{EquationNote|Eq.1}} requires {{mvar|N}} arithmetic operations per output value and {{math|N²}} operations for {{mvar|N}} outputs. That can be significantly reduced with any of several fast algorithms. Digital signal processing and other applications typically use fast convolution algorithms to reduce the cost of the convolution to O({{mvar|N}} log {{mvar|N}}) complexity.

The most common fast convolution algorithms use fast Fourier transform (FFT) algorithms via the circular convolution theorem. Specifically, the circular convolution of two finite-length sequences is found by taking an FFT of each sequence, multiplying pointwise, and then performing an inverse FFT. Convolutions of the type defined above are then efficiently implemented using that technique in conjunction with zero-extension and/or discarding portions of the output. Other fast convolution algorithms, such as the Schönhage–Strassen algorithm or the Mersenne transform,{{cite journal|last=Rader|first=C.M.|title=Discrete Convolutions via Mersenne Transforms|journal=IEEE Transactions on Computers|date=December 1972|volume=21|issue=12|pages=1269–1273|doi=10.1109/T-C.1972.223497|s2cid=1939809}} use fast Fourier transforms in other rings. The Winograd method is used as an alternative to the FFT.{{Cite book |last=Winograd |first=Shmuel |url=https://epubs.siam.org/doi/book/10.1137/1.9781611970364 |title=Arithmetic Complexity of Computations |date=January 1980 |publisher=Society for Industrial and Applied Mathematics |isbn=978-0-89871-163-9 |language=en |doi=10.1137/1.9781611970364}} It significantly speeds up 1D,{{Cite journal |last1=Lyakhov |first1=P. A. |last2=Nagornov |first2=N. N. |last3=Semyonova |first3=N. F. |last4=Abdulsalyamova |first4=A. S. |date=June 2023 |title=Reducing the Computational Complexity of Image Processing Using Wavelet Transform Based on the Winograd Method |url=https://link.springer.com/10.1134/S1054661823020074 |journal=Pattern Recognition and Image Analysis |language=en |volume=33 |issue=2 |pages=184–191 |doi=10.1134/S1054661823020074 |s2cid=259310351 |issn=1054-6618}} 2D,{{Cite journal |last1=Wu |first1=Di |last2=Fan |first2=Xitian |last3=Cao |first3=Wei |last4=Wang |first4=Lingli |date=May 2021 |title=SWM: A High-Performance Sparse-Winograd Matrix Multiplication CNN Accelerator |url=https://ieeexplore.ieee.org/document/9373543 |journal=IEEE Transactions on Very Large Scale Integration (VLSI) Systems |volume=29 |issue=5 |pages=936–949 |doi=10.1109/TVLSI.2021.3060041 |s2cid=233433757 |issn=1063-8210}} and 3D{{Cite journal |last1=Mittal |first1=Sparsh |last2=Vibhu |date=May 2021 |title=A survey of accelerator architectures for 3D convolution neural networks |url=https://linkinghub.elsevier.com/retrieve/pii/S1383762121000400 |journal=Journal of Systems Architecture |language=en |volume=115 |pages=102041 |doi=10.1016/j.sysarc.2021.102041|s2cid=233917781 }} convolution.

If one sequence is much longer than the other, zero-extension of the shorter sequence and fast circular convolution is not the most computationally efficient method available.{{cite book|editor-last=Madisetti |editor-first=Vijay K. |chapter=Fast Convolution and Filtering |first1=Ivan W. |last1=Selesnick |first2=C. Sidney |last2=Burrus |title=Digital Signal Processing Handbook |year=1999 |publisher=CRC Press |isbn=978-1-4200-4563-5 |page=Section 8}} Instead, decomposing the longer sequence into blocks and convolving each block allows for faster algorithms such as the overlap–save method and overlap–add method.{{cite web|last=Juang|first=B.H.|title=Lecture 21: Block Convolution|url=https://users.ece.gatech.edu/~juang/4270/BHJ4270-21.pdf |archive-url=https://web.archive.org/web/20040729204137/https://users.ece.gatech.edu/~juang/4270/BHJ4270-21.pdf |archive-date=2004-07-29 |url-status=live|publisher=EECS at the Georgia Institute of Technology|access-date=17 May 2013}} A hybrid convolution method that combines block and FIR algorithms allows for a zero input-output latency that is useful for real-time convolution computations.{{cite journal|last=Gardner|first=William G.|title=Efficient Convolution without Input/Output Delay|journal=Audio Engineering Society Convention 97|date=November 1994|series=Paper 3897|url=https://cs.ust.hk/mjg_lib/bibs/DPSu/DPSu.Files/Ga95.PDF |archive-url=https://web.archive.org/web/20150408211312/https://cs.ust.hk/mjg_lib/bibs/DPSu/DPSu.Files/Ga95.PDF |archive-date=2015-04-08 |url-status=live|access-date=17 May 2013}}

The convolution of two complex-valued functions on {{math|R^d}} is itself a complex-valued function on {{math|R^d}}, defined by:

:$(f\; *\; g\; )(x)\; =\; \backslash int\_\{\backslash mathbf\{R\}^d\}\; f(y)g(x-y)\backslash ,dy\; =\; \backslash int\_\{\backslash mathbf\{R\}^d\}\; f(x-y)g(y)\backslash ,dy,$

and is well-defined only if {{mvar|f}} and {{mvar|g}} decay sufficiently rapidly at infinity in order for the integral to exist. Conditions for the existence of the convolution may be tricky, since a blow-up in {{mvar|g}} at infinity can be easily offset by sufficiently rapid decay in {{mvar|f}}. The question of existence thus may involve different conditions on {{mvar|f}} and {{mvar|g}}:

If {{mvar|f}} and {{mvar|g}} are compactly supported continuous functions, then their convolution exists, and is also compactly supported and continuous {{harv|Hörmander|1983|loc=Chapter 1}}. More generally, if either function (say {{mvar|f}}) is compactly supported and the other is locally integrable, then the convolution {{math|f∗g}} is well-defined and continuous.

Convolution of {{mvar|f}} and {{mvar|g}} is also well defined when both functions are locally square integrable on {{math|R}} and supported on an interval of the form {{math|[a, +∞)}} (or both supported on {{math|[−∞, a]}}).

The convolution of {{mvar|f}} and {{mvar|g}} exists if {{mvar|f}} and {{mvar|g}} are both Lebesgue integrable functions in Lp space, and in this case {{math|f∗g}} is also integrable {{harv|Stein|Weiss|1971|loc=Theorem 1.3}}. This is a consequence of Tonelli's theorem. This is also true for functions in {{math|L¹}}, under the discrete convolution, or more generally for the convolution on any group.

Likewise, if {{math|f ∈ L¹}}({{math|R^d}})  and  {{math|g ∈ L^p}}({{math|R^d}})  where {{math|1 ≤ p ≤ ∞}},  then  {{math|f*g ∈ L^p}}({{math|R^d}}),  and

:$\backslash |\{f\}*\; g\backslash |\_p\backslash le\; \backslash |f\backslash |\_1\backslash |g\backslash |\_p.$

In the particular case {{math|p {{=}} 1}}, this shows that {{math|L¹}} is a Banach algebra under the convolution (and equality of the two sides holds if {{mvar|f}} and {{mvar|g}} are non-negative almost everywhere).

More generally, Young's inequality implies that the convolution is a continuous bilinear map between suitable {{math|L^p}} spaces. Specifically, if {{math| 1 ≤ p, q, r ≤ ∞}} satisfy:

:$\backslash frac\{1\}\{p\}+\backslash frac\{1\}\{q\}=\backslash frac\{1\}\{r\}+1,$

then

:$\backslash left\backslash Vert\; f*g\backslash right\backslash Vert\_r\backslash le\backslash left\backslash Vert\; f\backslash right\backslash Vert\_p\backslash left\backslash Vert\; g\backslash right\backslash Vert\_q,\backslash quad\; f\backslash in\; L^p,\backslash \; g\backslash in\; L^q,$

so that the convolution is a continuous bilinear mapping from {{math|L^p×L^q}} to {{math|L^r}}.

The Young inequality for convolution is also true in other contexts (circle group, convolution on {{math|Z}}). The preceding inequality is not sharp on the real line: when {{math| 1 < p, q, r < ∞}}, there exists a constant {{math|B_p,q < 1}} such that:

:$\backslash left\backslash Vert\; f*g\backslash right\backslash Vert\_r\backslash le\; B\_\{p,q\}\backslash left\backslash Vert\; f\backslash right\backslash Vert\_p\backslash left\backslash Vert\; g\backslash right\backslash Vert\_q,\backslash quad\; f\backslash in\; L^p,\backslash \; g\backslash in\; L^q.$

The optimal value of {{math|B_p,q}} was discovered in 1975{{cite journal

| first1=William | last1=Beckner | authorlink1=William Beckner (mathematician)

| year=1975

| title=Inequalities in Fourier analysis

| journal=Annals of Mathematics |series=Second Series

| volume=102

| issue=1 | pages=159–182

| doi=10.2307/1970980| jstor=1970980}} and independently in 1976,{{cite journal

| first1=Herm Jan | last1=Brascamp

| first2=Elliott H. | last2=Lieb | authorlink2=Elliott H. Lieb

| year=1976

| title=Best constants in Young's inequality, its converse, and its generalization to more than three functions

| journal=Advances in Mathematics

| volume=20

| issue=2

| pages=151–173

| doi=10.1016/0001-8708(76)90184-5 | doi-access=free}} see Brascamp–Lieb inequality.

A stronger estimate is true provided {{math| 1 < p, q, r < ∞}}:

:$\backslash |f\; *\; g\backslash |\_r\backslash le\; C\_\{p,q\}\backslash |f\backslash |\_p\backslash |g\backslash |\_\{q,w\}$

where $\backslash |g\backslash |\_\{q,w\}$ is the Lp space#Weak Lp norm. Convolution also defines a bilinear continuous map $L^\{p,w\}\backslash times\; L^\{q,w\}\backslash to\; L^\{r,w\}$ for , owing to the weak Young inequality:{{harvnb|Reed|Simon|1975|loc=IX.4}}

:$\backslash |f\; *\; g\backslash |\_\{r,w\}\backslash le\; C\_\{p,q\}\backslash |f\backslash |\_\{p,w\}\backslash |g\backslash |\_\{r,w\}.$

In addition to compactly supported functions and integrable functions, functions that have sufficiently rapid decay at infinity can also be convolved. An important feature of the convolution is that if f and g both decay rapidly, then f∗g also decays rapidly. In particular, if f and g are rapidly decreasing functions, then so is the convolution f∗g. Combined with the fact that convolution commutes with differentiation (see #Properties), it follows that the class of Schwartz functions is closed under convolution {{harv|Stein|Weiss|1971|loc=Theorem 3.3}}.

{{Main article|Distribution (mathematics)}}

If f is a smooth function that is compactly supported and g is a distribution, then f∗g is a smooth function defined by

:$\backslash int\_\{\backslash mathbb\{R\}^d\}\; \{f\}(y)g(x-y)\backslash ,dy\; =\; (f*g)(x)\; \backslash in\; C^\backslash infty(\backslash mathbb\{R\}^d)\; .$

More generally, it is possible to extend the definition of the convolution in a unique way with $\backslash varphi$ the same as f above, so that the associative law

:$f*\; (g*\; \backslash varphi)\; =\; (f*\; g)*\; \backslash varphi$

remains valid in the case where f is a distribution, and g a compactly supported distribution {{harv|Hörmander|1983|loc=§4.2}}.

The convolution of any two Borel measures μ and ν of bounded variation is the measure $\backslash mu*\backslash nu$ defined by {{harv|Rudin|1962}}

:$\backslash int\_\{\backslash mathbf\{R\}^d\}\; f(x)\; \backslash ,\; d(\backslash mu*\backslash nu)(x)\; =\; \backslash int\_\{\backslash mathbf\{R\}^d\}\backslash int\_\{\backslash mathbf\{R\}^d\}f(x+y)\backslash ,d\backslash mu(x)\backslash ,d\backslash nu(y).$

In particular,

: $(\backslash mu*\backslash nu)(A)\; =\; \backslash int\_\{\backslash mathbf\{R\}^d\backslash times\backslash mathbf\; R^d\}1\_A(x+y)\backslash ,\; d(\backslash mu\backslash times\backslash nu)(x,y),$

where $A\backslash subset\backslash mathbf\; R^d$ is a measurable set and $1\_A$ is the indicator function of $A$.

This agrees with the convolution defined above when μ and ν are regarded as distributions, as well as the convolution of L¹ functions when μ and ν are absolutely continuous with respect to the Lebesgue measure.

The convolution of measures also satisfies the following version of Young's inequality

:$\backslash |\backslash mu*\; \backslash nu\backslash |\backslash le\; \backslash |\backslash mu\backslash |\backslash |\backslash nu\backslash |$

where the norm is the total variation of a measure. Because the space of measures of bounded variation is a Banach space, convolution of measures can be treated with standard methods of functional analysis that may not apply for the convolution of distributions.

{{See also|Convolution algebra}}

The convolution defines a product on the linear space of integrable functions. This product satisfies the following algebraic properties, which formally mean that the space of integrable functions with the product given by convolution is a commutative associative algebra without identity {{harv|Strichartz|1994|loc=§3.3}}. Other linear spaces of functions, such as the space of continuous functions of compact support, are closed under the convolution, and so also form commutative associative algebras.

; Commutativity: $$f\; *\; g\; =\; g\; *\; f$$ Proof: By definition: $$(f\; *\; g)(t)\; =\; \backslash int^\backslash infty\_\{-\backslash infty\}\; f(\backslash tau)g(t\; -\; \backslash tau)\backslash ,\; d\backslash tau$$ Changing the variable of integration to $u\; =\; t\; -\; \backslash tau$ the result follows.

; Associativity: $$f\; *\; (g\; *\; h)\; =\; (f\; *\; g)\; *\; h$$ Proof: This follows from using Fubini's theorem (i.e., double integrals can be evaluated as iterated integrals in either order).

; Distributivity: $$f\; *\; (g\; +\; h)\; =\; (f\; *\; g)\; +\; (f\; *\; h)$$ Proof: This follows from linearity of the integral.

; Associativity with scalar multiplication: $$a\; (f\; *\; g)\; =\; (a\; f)\; *\; g$$ for any real (or complex) number $a$.

; Multiplicative identity: No algebra of functions possesses an identity for the convolution. The lack of identity is typically not a major inconvenience, since most collections of functions on which the convolution is performed can be convolved with a delta distribution (a unitary impulse, centered at zero) or, at the very least (as is the case of L¹) admit approximations to the identity. The linear space of compactly supported distributions does, however, admit an identity under the convolution. Specifically, $$f\; *\; \backslash delta\; =\; f$$ where δ is the delta distribution.

; Inverse element: Some distributions S have an inverse element S⁻¹ for the convolution which then must satisfy $$S^\{-1\}\; *\; S\; =\; \backslash delta$$ from which an explicit formula for S⁻¹ may be obtained.{{paragraph}}The set of invertible distributions forms an abelian group under the convolution.

; Complex conjugation: $$\backslash overline\{f\; *\; g\}\; =\; \backslash overline\{f\}\; *\; \backslash overline\{g\}$$

; Time reversal: If  $q(t)\; =\; r(t)*s(t),$  then  $q(-t)\; =\; r(-t)*s(-t).$

Proof (using convolution theorem):

$q(t)\; \backslash \; \backslash stackrel\{\backslash mathcal\{F\}\}\{\backslash Longleftrightarrow\}\backslash \; \backslash \; Q(f)\; =\; R(f)S(f)$

$q(-t)\; \backslash \; \backslash stackrel\{\backslash mathcal\{F\}\}\{\backslash Longleftrightarrow\}\backslash \; \backslash \; Q(-f)\; =\; R(-f)S(-f)$

$$

\begin{align}

q(-t) &= \mathcal{F}^{-1}\bigg\{R(-f)S(-f)\bigg\}\\

&= \mathcal{F}^{-1}\bigg\{R (-f)\bigg\} * \mathcal{F}^{-1}\bigg\{S(-f)\bigg\}\\

&= r(-t) * s(-t)

\end{align}

; Relationship with differentiation: $$(f\; *\; g)\text{'}\; =\; f\text{'}\; *\; g\; =\; f\; *\; g\text{'}$$ Proof:

:$$

\begin{align}

(f * g)' & = \frac{d}{dt} \int^\infty_{-\infty} f(\tau) g(t - \tau) \, d\tau \\

& =\int^\infty_{-\infty} f(\tau) \frac{\partial}{\partial t} g(t - \tau) \, d\tau \\

& =\int^\infty_{-\infty} f(\tau) g'(t - \tau) \, d\tau = f* g'.

\end{align}

; Relationship with integration: If $F(t)\; =\; \backslash int^t\_\{-\backslash infty\}\; f(\backslash tau)\; d\backslash tau,$ and $G(t)\; =\; \backslash int^t\_\{-\backslash infty\}\; g(\backslash tau)\; \backslash ,\; d\backslash tau,$ then $$(F\; *\; g)(t)\; =\; (f\; *\; G)(t)\; =\; \backslash int^t\_\{-\backslash infty\}(f\; *\; g)(\backslash tau)\backslash ,d\backslash tau.$$

If f and g are integrable functions, then the integral of their convolution on the whole space is simply obtained as the product of their integrals:{{Cite web|last=Weisstein|first=Eric W.|title=Convolution|url=https://mathworld.wolfram.com/Convolution.html|access-date=2021-09-22|website=mathworld.wolfram.com|language=en}}

: $\backslash int\_\{\backslash mathbf\{R\}^d\}(f\; *\; g)(x)\; \backslash ,\; dx=\backslash left(\backslash int\_\{\backslash mathbf\{R\}^d\}f(x)\; \backslash ,\; dx\backslash right)\; \backslash left(\backslash int\_\{\backslash mathbf\{R\}^d\}g(x)\; \backslash ,\; dx\backslash right).$

This follows from Fubini's theorem. The same result holds if f and g are only assumed to be nonnegative measurable functions, by Tonelli's theorem.

In the one-variable case,

: $\backslash frac\{d\}\{dx\}(f\; *\; g)\; =\; \backslash frac\{df\}\{dx\}\; *\; g\; =\; f\; *\; \backslash frac\{dg\}\{dx\}$

where $\backslash frac\{d\}\{dx\}$ is the derivative. More generally, in the case of functions of several variables, an analogous formula holds with the partial derivative:

: $\backslash frac\{\backslash partial\}\{\backslash partial\; x\_i\}(f\; *\; g)\; =\; \backslash frac\{\backslash partial\; f\}\{\backslash partial\; x\_i\}\; *\; g\; =\; f\; *\; \backslash frac\{\backslash partial\; g\}\{\backslash partial\; x\_i\}.$

A particular consequence of this is that the convolution can be viewed as a "smoothing" operation: the convolution of f and g is differentiable as many times as f and g are in total.

These identities hold for example under the condition that f and g are absolutely integrable and at least one of them has an absolutely integrable (L¹) weak derivative, as a consequence of Young's convolution inequality. For instance, when f is continuously differentiable with compact support, and g is an arbitrary locally integrable function,

: $\backslash frac\{d\}\{dx\}(f*\; g)\; =\; \backslash frac\{df\}\{dx\}\; *\; g.$

These identities also hold much more broadly in the sense of tempered distributions if one of f or g is a

rapidly decreasing tempered distribution, a

compactly supported tempered distribution or a Schwartz function and the other is a tempered distribution. On the other hand, two positive integrable and infinitely differentiable functions may have a nowhere continuous convolution.

In the discrete case, the difference operator D f(n) = f(n + 1) − f(n) satisfies an analogous relationship:

: $D(f\; *\; g)\; =\; (Df)\; *\; g\; =\; f\; *\; (Dg).$

The convolution theorem states that{{cite web |last1=Weisstein |first1=Eric W |title=From MathWorld--A Wolfram Web Resource |url=https://mathworld.wolfram.com/ConvolutionTheorem.html}}

: $\backslash mathcal\{F\}\backslash \{f\; *\; g\backslash \}\; =\; \backslash mathcal\{F\}\backslash \{f\backslash \}\backslash cdot\; \backslash mathcal\{F\}\backslash \{g\backslash \}$

where $\backslash mathcal\{F\}\backslash \{f\backslash \}$ denotes the Fourier transform of $f$.

Versions of this theorem also hold for the Laplace transform, two-sided Laplace transform, Z-transform and Mellin transform.

If $\backslash mathcal\; W$ is the Fourier transform matrix, then

: $\backslash mathcal\; W\backslash left(C^\{(1)\}x\; \backslash ast\; C^\{(2)\}y\backslash right)\; =\; \backslash left(\backslash mathcal\; W\; C^\{(1)\}\; \backslash bull\; \backslash mathcal\; W\; C^\{(2)\}\backslash right)(x\; \backslash otimes\; y)\; =\; \backslash mathcal\; W\; C^\{(1)\}x\; \backslash circ\; \backslash mathcal\; W\; C^\{(2)\}y$,

where $\backslash bull$ is face-splitting product,{{Cite journal|last=Slyusar|first=V. I.|date= December 27, 1996|title=End products in matrices in radar applications. |url=https://slyusar.kiev.ua/en/IZV_1998_3.pdf |archive-url=https://web.archive.org/web/20130811122444/https://slyusar.kiev.ua/en/IZV_1998_3.pdf |archive-date=2013-08-11 |url-status=live|journal=Radioelectronics and Communications Systems |volume=41 |issue=3|pages=50–53}}{{Cite journal|last=Slyusar|first=V. I.|date=1997-05-20|title=Analytical model of the digital antenna array on a basis of face-splitting matrix products. |url=https://slyusar.kiev.ua/ICATT97.pdf |archive-url=https://web.archive.org/web/20130811112059/https://slyusar.kiev.ua/ICATT97.pdf |archive-date=2013-08-11 |url-status=live|journal=Proc. ICATT-97, Kyiv|pages=108–109}}{{Cite journal|last=Slyusar|first=V. I.|date=1997-09-15|title=New operations of matrices product for applications of radars|url=https://slyusar.kiev.ua/DIPED_1997.pdf |archive-url=https://web.archive.org/web/20130811113217/https://slyusar.kiev.ua/DIPED_1997.pdf |archive-date=2013-08-11 |url-status=live|journal=Proc. Direct and Inverse Problems of Electromagnetic and Acoustic Wave Theory (DIPED-97), Lviv.|pages=73–74}}{{Cite journal|last=Slyusar|first=V. I.|date=March 13, 1998|title=A Family of Face Products of Matrices and its Properties|url=https://slyusar.kiev.ua/FACE.pdf |archive-url=https://web.archive.org/web/20130811113935/https://slyusar.kiev.ua/FACE.pdf |archive-date=2013-08-11 |url-status=live|journal=Cybernetics and Systems Analysis C/C of Kibernetika I Sistemnyi Analiz.- 1999.|volume=35|issue=3|pages=379–384|doi=10.1007/BF02733426|s2cid=119661450}}{{Cite journal|last=Slyusar|first=V. I.|date=2003|title=Generalized face-products of matrices in models of digital antenna arrays with nonidentical channels|url=https://slyusar.kiev.ua/en/IZV_2003_10.pdf |archive-url=https://web.archive.org/web/20130811125643/https://slyusar.kiev.ua/en/IZV_2003_10.pdf |archive-date=2013-08-11 |url-status=live|journal=Radioelectronics and Communications Systems|volume=46|issue=10|pages=9–17}} $\backslash otimes$ denotes Kronecker product, $\backslash circ$ denotes Hadamard product (this result is an evolving of count sketch properties{{cite conference

| title = Fast and scalable polynomial kernels via explicit feature maps

| last1 = Ninh | first1 = Pham

| first2 = Rasmus | last2 = Pagh | author2-link = Rasmus Pagh

| date = 2013

| publisher = Association for Computing Machinery

| conference = SIGKDD international conference on Knowledge discovery and data mining

| doi = 10.1145/2487575.2487591

}}).

This can be generalized for appropriate matrices $\backslash mathbf\{A\},\backslash mathbf\{B\}$:

: $\backslash mathcal\; W\backslash left((\backslash mathbf\{A\}x)\; \backslash ast\; (\backslash mathbf\{B\}y)\backslash right)\; =\; \backslash left((\backslash mathcal\; W\; \backslash mathbf\{A\})\; \backslash bull\; (\backslash mathcal\; W\; \backslash mathbf\{B\})\backslash right)(x\; \backslash otimes\; y)\; =\; (\backslash mathcal\; W\; \backslash mathbf\{A\}x)\; \backslash circ\; (\backslash mathcal\; W\; \backslash mathbf\{B\}y)$

from the properties of the face-splitting product.

The convolution commutes with translations, meaning that

: $\backslash tau\_x\; (f\; *\; g)\; =\; (\backslash tau\_x\; f)\; *\; g\; =\; f\; *\; (\backslash tau\_x\; g)$

where τ_xf is the translation of the function f by x defined by

: $(\backslash tau\_x\; f)(y)\; =\; f(y\; -\; x).$

If f is a Schwartz function, then τ_xf is the convolution with a translated Dirac delta function τ_xf = f ∗ τ_x δ. So translation invariance of the convolution of Schwartz functions is a consequence of the associativity of convolution.

Furthermore, under certain conditions, convolution is the most general translation invariant operation. Informally speaking, the following holds

: Suppose that S is a bounded linear operator acting on functions which commutes with translations: S(τ_xf) = τ_x(Sf) for all x. Then S is given as convolution with a function (or distribution) g_S; that is Sf = g_S ∗ f.

Thus some translation invariant operations can be represented as convolution. Convolutions play an important role in the study of time-invariant systems, and especially LTI system theory. The representing function g_S is the impulse response of the transformation S.

A more precise version of the theorem quoted above requires specifying the class of functions on which the convolution is defined, and also requires assuming in addition that S must be a continuous linear operator with respect to the appropriate topology. It is known, for instance, that every continuous translation invariant continuous linear operator on L¹ is the convolution with a finite Borel measure. More generally, every continuous translation invariant continuous linear operator on L^p for 1 ≤ p < ∞ is the convolution with a tempered distribution whose Fourier transform is bounded. To wit, they are all given by bounded Fourier multipliers.

If G is a suitable group endowed with a measure λ, and if f and g are real or complex valued integrable functions on G, then we can define their convolution by

:$(f\; *\; g)(x)\; =\; \backslash int\_G\; f(y)\; g\backslash left(y^\{-1\}x\backslash right)\backslash ,d\backslash lambda(y).$

It is not commutative in general. In typical cases of interest G is a locally compact Hausdorff topological group and λ is a (left-) Haar measure. In that case, unless G is unimodular, the convolution defined in this way is not the same as $\backslash int\; f\backslash left(xy^\{-1\}\backslash right)g(y)\; \backslash ,\; d\backslash lambda(y)$. The preference of one over the other is made so that convolution with a fixed function g commutes with left translation in the group:

:$L\_h(f*\; g)\; =\; (L\_hf)*\; g.$

Furthermore, the convention is also required for consistency with the definition of the convolution of measures given below. However, with a right instead of a left Haar measure, the latter integral is preferred over the former.

On locally compact abelian groups, a version of the convolution theorem holds: the Fourier transform of a convolution is the pointwise product of the Fourier transforms. The circle group T with the Lebesgue measure is an immediate example. For a fixed g in L¹(T), we have the following familiar operator acting on the Hilbert space L²(T):

:$T\; \{f\}(x)\; =\; \backslash frac\{1\}\{2\; \backslash pi\}\; \backslash int\_\{\backslash mathbf\{T\}\}\; \{f\}(y)\; g(\; x\; -\; y)\; \backslash ,\; dy.$

The operator T is compact. A direct calculation shows that its adjoint T* is convolution with

:$\backslash bar\{g\}(-y).$

By the commutativity property cited above, T is normal: T* T = TT* . Also, T commutes with the translation operators. Consider the family S of operators consisting of all such convolutions and the translation operators. Then S is a commuting family of normal operators. According to spectral theory, there exists an orthonormal basis {h_k} that simultaneously diagonalizes S. This characterizes convolutions on the circle. Specifically, we have

:$h\_k\; (x)\; =\; e^\{ikx\},\; \backslash quad\; k\; \backslash in\; \backslash mathbb\{Z\},\backslash ;$

which are precisely the characters of T. Each convolution is a compact multiplication operator in this basis. This can be viewed as a version of the convolution theorem discussed above.

A discrete example is a finite cyclic group of order n. Convolution operators are here represented by circulant matrices, and can be diagonalized by the discrete Fourier transform.

A similar result holds for compact groups (not necessarily abelian): the matrix coefficients of finite-dimensional unitary representations form an orthonormal basis in L² by the Peter–Weyl theorem, and an analog of the convolution theorem continues to hold, along with many other aspects of harmonic analysis that depend on the Fourier transform.

Let G be a (multiplicatively written) topological group.

If μ and ν are Radon measures on G, then their convolution μ∗ν is defined as the pushforward measure of the group action and can be written asHewitt and Ross (1979) Abstract harmonic analysis, volume 1, second edition, Springer-Verlag, p 266.

:$(\backslash mu\; *\; \backslash nu)(E)\; =\; \backslash iint\; 1\_E(xy)\; \backslash ,d\backslash mu(x)\; \backslash ,d\backslash nu(y)$

for each measurable subset E of G. The convolution is also a Radon measure, whose total variation satisfies

:$\backslash |\backslash mu\; *\; \backslash nu\backslash |\; \backslash le\; \backslash left\backslash |\backslash mu\backslash right\backslash |\; \backslash left\backslash |\backslash nu\backslash right\backslash |.$

In the case when G is locally compact with (left-)Haar measure λ, and μ and ν are absolutely continuous with respect to a λ, so that each has a density function, then the convolution μ∗ν is also absolutely continuous, and its density function is just the convolution of the two separate density functions. In fact, if either measure is absolutely continuous with respect to the Haar measure, then so is their convolution.Hewitt and Ross (1979), Theorem 19.18, p 272.

If μ and ν are probability measures on the topological group {{nowrap|(R,+),}} then the convolution μ∗ν is the probability distribution of the sum X + Y of two independent random variables X and Y whose respective distributions are μ and ν.

In convex analysis, the infimal convolution of proper (not identically $+\backslash infty$) convex functions $f\_1,\backslash dots,f\_m$ on $\backslash mathbb\; R^n$ is defined by:{{citation|author=R. Tyrrell Rockafellar|title=Convex analysis|publisher=Princeton University Press|year=1970}}

$$(f\_1*\backslash cdots*f\_m)(x)=\backslash inf\_x\; \backslash \{\; f\_1(x\_1)+\backslash cdots+f\_m(x\_m)\; |\; x\_1+\backslash cdots+x\_m\; =\; x\backslash \}.$$

It can be shown that the infimal convolution of convex functions is convex. Furthermore, it satisfies an identity analogous to that of the Fourier transform of a traditional convolution, with the role of the Fourier transform is played instead by the Legendre transform:

$$\backslash varphi^*(x)\; =\; \backslash sup\_y\; (\; x\backslash cdot\; y\; -\; \backslash varphi(y)).$$

We have:

$$(f\_1*\backslash cdots\; *f\_m)^*(x)\; =\; f\_1^*(x)\; +\; \backslash cdots\; +\; f\_m^*(x).$$

Let (X, Δ, ∇, ε, η) be a bialgebra with comultiplication Δ, multiplication ∇, unit η, and counit ε. The convolution is a product defined on the endomorphism algebra End(X) as follows. Let φ, ψ ∈ End(X), that is, φ, ψ: X → X are functions that respect all algebraic structure of X, then the convolution φ∗ψ is defined as the composition

:$X\; \backslash mathrel\{\backslash xrightarrow\{\backslash Delta\}\}\; X\; \backslash otimes\; X\; \backslash mathrel\{\backslash xrightarrow\{\backslash phi\backslash otimes\backslash psi\}\}\; X\; \backslash otimes\; X\; \backslash mathrel\{\backslash xrightarrow\{\backslash nabla\}\}\; X.$

The convolution appears notably in the definition of Hopf algebras {{harv|Kassel|1995|loc=§III.3}}. A bialgebra is a Hopf algebra if and only if it has an antipode: an endomorphism S such that

:$S\; *\; \backslash operatorname\{id\}\_X\; =\; \backslash operatorname\{id\}\_X\; *\; S\; =\; \backslash eta\backslash circ\backslash varepsilon.$

File:Halftone, Gaussian Blur.jpg can be used to obtain a smooth grayscale digital image of a halftone print.]]

Convolution and related operations are found in many applications in science, engineering and mathematics.

• Convolutional neural networks apply multiple cascaded convolution kernels with applications in machine vision and artificial intelligence.{{Cite journal|last1=Zhang|first1=Yingjie|last2=Soon|first2=Hong Geok|last3=Ye|first3=Dongsen|last4=Fuh|first4=Jerry Ying Hsi|last5=Zhu|first5=Kunpeng|date=September 2020|title=Powder-Bed Fusion Process Monitoring by Machine Vision With Hybrid Convolutional Neural Networks|url=https://ieeexplore.ieee.org/document/8913613|journal=IEEE Transactions on Industrial Informatics|volume=16|issue=9|pages=5769–5779|doi=10.1109/TII.2019.2956078|s2cid=213010088|issn=1941-0050}}{{Cite journal|last1=Chervyakov|first1=N.I.|last2=Lyakhov|first2=P.A.|last3=Deryabin|first3=M.A.|last4=Nagornov|first4=N.N.|last5=Valueva|first5=M.V.|last6=Valuev|first6=G.V.|date=September 2020|title=Residue Number System-Based Solution for Reducing the Hardware Cost of a Convolutional Neural Network|url=https://linkinghub.elsevier.com/retrieve/pii/S092523122030583X|journal=Neurocomputing|language=en|volume=407|pages=439–453|doi=10.1016/j.neucom.2020.04.018|s2cid=219470398|quote=Convolutional neural networks represent deep learning architectures that are currently used in a wide range of applications, including computer vision, speech recognition, time series analysis in finance, and many others.}} Though these are actually cross-correlations rather than convolutions in most cases.{{Cite journal|last=Atlas, Homma, and Marks|title=An Artificial Neural Network for Spatio-Temporal Bipolar Patterns: Application to Phoneme Classification|url=https://papers.nips.cc/paper/1987/file/98f13708210194c475687be6106a3b84-Paper.pdf |archive-url=https://web.archive.org/web/20210414091306/https://papers.nips.cc/paper/1987/file/98f13708210194c475687be6106a3b84-Paper.pdf |archive-date=2021-04-14 |url-status=live|journal=Neural Information Processing Systems (NIPS 1987)|volume=1}}
• In non-neural-network-based image processing
• In digital image processing convolutional filtering plays an important role in many important algorithms in edge detection and related processes (see Kernel (image processing))
• In optics, an out-of-focus photograph is a convolution of the sharp image with a lens function. The photographic term for this is bokeh.
• In image processing applications such as adding blurring.
• In digital data processing
• In analytical chemistry, Savitzky–Golay smoothing filters are used for the analysis of spectroscopic data. They can improve signal-to-noise ratio with minimal distortion of the spectra
• In statistics, a weighted moving average is a convolution.
• In acoustics, reverberation is the convolution of the original sound with echoes from objects surrounding the sound source.
• In digital signal processing, convolution is used to map the impulse response of a real room on a digital audio signal.
• In electronic music convolution is the imposition of a spectral or rhythmic structure on a sound. Often this envelope or structure is taken from another sound. The convolution of two signals is the filtering of one through the other.Zölzer, Udo, ed. (2002). DAFX:Digital Audio Effects, p.48–49. {{isbn|0471490784}}.
• In electrical engineering, the convolution of one function (the input signal) with a second function (the impulse response) gives the output of a linear time-invariant system (LTI). At any given moment, the output is an accumulated effect of all the prior values of the input function, with the most recent values typically having the most influence (expressed as a multiplicative factor). The impulse response function provides that factor as a function of the elapsed time since each input value occurred.
• In physics, wherever there is a linear system with a "superposition principle", a convolution operation makes an appearance. For instance, in spectroscopy line broadening due to the Doppler effect on its own gives a Gaussian spectral line shape and collision broadening alone gives a Lorentzian line shape. When both effects are operative, the line shape is a convolution of Gaussian and Lorentzian, a Voigt function.
• In time-resolved fluorescence spectroscopy, the excitation signal can be treated as a chain of delta pulses, and the measured fluorescence is a sum of exponential decays from each delta pulse.
• In computational fluid dynamics, the large eddy simulation (LES) turbulence model uses the convolution operation to lower the range of length scales necessary in computation thereby reducing computational cost.
• In probability theory, the probability distribution of the sum of two independent random variables is the convolution of their individual distributions.
• In kernel density estimation, a distribution is estimated from sample points by convolution with a kernel, such as an isotropic Gaussian.{{sfn|Diggle|1985}}
• In radiotherapy treatment planning systems, most part of all modern codes of calculation applies a convolution-superposition algorithm.{{Clarify|date=May 2013}}
• In structural reliability, the reliability index can be defined based on the convolution theorem.
• The definition of reliability index for limit state functions with nonnormal distributions can be established corresponding to the joint distribution function. In fact, the joint distribution function can be obtained using the convolution theory.{{sfn|Ghasemi|Nowak|2017}}
• In Smoothed-particle hydrodynamics, simulations of fluid dynamics are calculated using particles, each with surrounding kernels. For any given particle $i$, some physical quantity $A\_i$ is calculated as a convolution of $A\_j$ with a weighting function, where $j$ denotes the neighbors of particle $i$: those that are located within its kernel. The convolution is approximated as a summation over each neighbor.{{cite journal |last1=Monaghan |first1=J. J. |title=Smoothed particle hydrodynamics |journal=Annual Review of Astronomy and Astrophysics |date=1992 |volume=30 |pages=543–547 |doi=10.1146/annurev.aa.30.090192.002551 |bibcode=1992ARA&A..30..543M |url=https://ui.adsabs.harvard.edu/abs/1992ARA&A..30..543M |access-date=16 February 2021 |ref=1992ARA&A..30..543M}}
• In Fractional calculus convolution is instrumental in various definitions of fractional integral and fractional derivative.

{{Div col|colwidth=30em}}

• Analog signal processing
• Circulant matrix
• Convolution for optical broad-beam responses in scattering media
• Convolution power
• Convolution quotient
• Deconvolution
• Dirichlet convolution
• Generalized signal averaging
• List of convolutions of probability distributions
• LTI system theory#Impulse response and convolution
• Multidimensional discrete convolution
• Scaled correlation
• Titchmarsh convolution theorem
• Toeplitz matrix (convolutions can be considered a Toeplitz matrix operation where each row is a shifted copy of the convolution kernel)
• Wavelet transform

{{Div col end}}

{{notelist-ua}}

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• {{Trèves François Topological vector spaces, distributions and kernels}}
• {{citation | last=Uludag | first=A. M. |author-link=A. Muhammed Uludag|title=On possible deterioration of smoothness under the operation of convolution|journal=J. Math. Anal. Appl. |volume=227 |issue=2 |pages=335–358|year=1998|doi=10.1006/jmaa.1998.6091 |doi-access=free|hdl=11693/25385 |hdl-access=free }}
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{{Wiktionary|convolution}}

{{Commons category|Convolution}}

• [https://jeff560.tripod.com/c.html Earliest Uses: The entry on Convolution has some historical information.]
• [https://web.archive.org/web/20060221234856/https://rkb.home.cern.ch/rkb/AN16pp/node38.html#SECTION000380000000000000000 Convolution], on [https://web.archive.org/web/20060512020859/https://rkb.home.cern.ch/rkb/titleA.html The Data Analysis BriefBook]
• https://jhu.edu/~signals/convolve/index.html Visual convolution Java Applet
• https://jhu.edu/~signals/discreteconv2/index.html Visual convolution Java Applet for discrete-time functions
• https://get-the-solution.net/projects/discret-convolution discret-convolution online calculator
• https://lpsa.swarthmore.edu/Convolution/CI.html Convolution demo and visualization in JavaScript
• https://phiresky.github.io/convolution-demo/ Another convolution demo in JavaScript
• Lectures on Image Processing: A collection of 18 lectures in pdf format from Vanderbilt University. Lecture 7 is on 2-D convolution., by Alan Peters
• https://archive.org/details/Lectures_on_Image_Processing
• [https://micro.magnet.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/ Convolution Kernel Mask Operation Interactive tutorial]
• [https://mathworld.wolfram.com/Convolution.html Convolution] at MathWorld
• [https://nongnu.org/freeverb3/ Freeverb3 Impulse Response Processor]: Opensource zero latency impulse response processor with VST plugins
• Stanford University CS 178 [https://graphics.stanford.edu/courses/cs178/applets/convolution.html interactive Flash demo] showing how spatial convolution works.
• [https://youtube.com/watch?v=IW4Reburjpc A video lecture on the subject of convolution] given by Salman Khan
• [https://dspguide.com/ch24/6.htm Example of FFT convolution for pattern-recognition (image processing)]
• [https://betterexplained.com/articles/intuitive-convolution/ Intuitive Guide to Convolution] A blogpost about an intuitive interpretation of convolution.

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