Silverman–Toeplitz theorem

In mathematics, the Silverman–Toeplitz theorem, first proved by Otto Toeplitz, is a result in series summability theory characterizing matrix summability methods that are regular. A regular matrix summability method is a linear sequence transformation that preserves the limits of convergent sequences.[https://archive.org/details/silvermantoeplit00rude Silverman–Toeplitz theorem], by Ruder, Brian, Published 1966, Call number LD2668 .R4 1966 R915, Publisher Kansas State University, Internet Archive The linear sequence transformation can be applied to the divergent sequences of partial sums of divergent series to give those series generalized sums.

An infinite matrix $(a_{i,j})_{i,j \in \mathbb{N}}$ with complex-valued entries defines a regular matrix summability method if and only if it satisfies all of the following properties:

: $\begin{align}
& \lim_{i \to \infty} a_{i,j} = 0 \quad j \in \mathbb{N} & & \text{(Every column sequence converges to 0.)} \\[3pt]
& \lim_{i \to \infty} \sum_{j=0}^{\infty} a_{i,j} = 1 & & \text{(The row sums converge to 1.)} \\[3pt]
& \sup_i \sum_{j=0}^{\infty} \vert a_{i,j} \vert < \infty & & \text{(The absolute row sums are bounded.)}
\end{align}$

An example is Cesàro summation, a matrix summability method with

: $a_{mn}=\begin{cases}\frac{1}{m} & n\le m\\ 0 & n>m\end{cases} = \begin{pmatrix}
1 & 0 & 0 & 0 & 0 & \cdots \\
\frac{1}{2} & \frac{1}{2} & 0 & 0 & 0 & \cdots \\
\frac{1}{3} & \frac{1}{3} & \frac{1}{3} & 0 & 0 & \cdots \\
\frac{1}{4} & \frac{1}{4} & \frac{1}{4} & \frac{1}{4} & 0 & \cdots \\
\frac{1}{5} & \frac{1}{5} & \frac{1}{5} & \frac{1}{5} & \frac{1}{5} & \cdots \\
\vdots & \vdots & \vdots & \vdots & \vdots & \ddots \\
\end{pmatrix}.$

Formal statement

Let the aforementioned inifinite matrix $(a_{i,j})_{i,j \in \mathbb{N}}$ of complex elements satisfy the following conditions:

\lim_{i \to \infty} a_{i,j} = 0 for every fixed

j \in \mathbb{N}

$\sup_{i \in \mathbb{N}} \sum_{j=1}^{i} \vert a_{i,j} \vert < \infty$ ;

and $z_{n}$ be a sequence of complex numbers that converges to $\lim_{n \to \infty} z_{n} = z_{\infty}$ . We denote $S_{n}$ as the weighted sum sequence: $S_{n} = \sum_{m = 1}^{n} { \left( a_{n, m} z_{n} \right) }$ .

Then the following results hold:

\lim_{n \to \infty} z_{n} = z_{\infty} = 0 , then

\lim_{n \to \infty} {S_{n}} = 0

\lim_{n \to \infty} z_{n} = z_{\infty} \ne 0 and

\lim_{i \to \infty} \sum_{j=1}^{i} a_{i,j} = 1

, then

\lim_{n \to \infty} {S_{n}} = z_{\infty}

.{{Cite journal |last1=Linero |first1=Antonio |last2=Rosalsky |first2=Andrew |date=2013-07-01 |title=On the Toeplitz Lemma, Convergence in Probability, and Mean Convergence |url=https://web.archive.org/web/20151203145937id_/http://stat.fsu.edu:80/~arlinero/assets/documents/Rosalsky-Toeplitz.pdf |journal=Stochastic Analysis and Applications |language=en |volume=31 |issue=4 |pages=684–694 |doi=10.1080/07362994.2013.799406 |issn=0736-2994 |access-date=2024-11-17}}

Proof

= Proving 1. =

For the fixed $j \in \mathbb{N}$ the complex sequences $z_{n}$ , $S_{n}$ and $a_{i, j}$ approach zero if and only if the real-values sequences $\left| z_{n} \right|$ , $\left| S_{n} \right|$ and $\left| a_{i, j} \right|$ approach zero respectively. We also introduce $M = \sup_{i \in \mathbb{N}} \sum_{j=1}^{i} \vert a_{i,j} \vert > 0$ .

Since $\left| z_{n} \right| \to 0$ , for prematurely chosen $\varepsilon > 0$ there exists $N_{\varepsilon} = N_{\varepsilon}\left( \varepsilon \right )$ , so for every $n > N_{\varepsilon}\left( \varepsilon \right )$ we have $\left| z_{n} \right| < \frac {\varepsilon} {2M}$ . Next, for some $N_{a} = N_{a}\left( \varepsilon \right ) > N_{\varepsilon}\left( \varepsilon \right )$ it's true, that $\left| a_{n, m} \right| < \frac {M} {N_{\varepsilon}}$ for every $n > N_{a}\left( \varepsilon \right )$ and $1 \leqslant m \leqslant n$ . Therefore, for every $n > N_{a}\left( \varepsilon \right )$

$\begin{align}
& \left| S_{n} \right|
= \left| \sum_{m = 1}^{n} \left( a_{n, m} z_{n} \right) \right| \leqslant \sum_{m = 1}^{n} \left( \left| a_{n, m} \right| \cdot \left| z_{n} \right| \right)
= \sum_{m = 1}^{N_{\varepsilon}} \left( \left| a_{n, m} \right| \cdot \left| z_{n} \right| \right) + \sum_{m = N_{\varepsilon}}^{n} \left( \left| a_{n, m} \right| \cdot \left| z_{n} \right| \right) < \\
& < N_{\varepsilon} \cdot \frac {M} {N_{\varepsilon}} \cdot \frac {\varepsilon} {2M} + \frac {\varepsilon} {2M} \sum_{m = N_{\varepsilon}}^{n} \left| a_{n, m} \right|
\leqslant \frac {\varepsilon} {2} + \frac {\varepsilon} {2M} \sum_{m = 1}^{n} \left| a_{n, m} \right|
\leqslant \frac {\varepsilon} {2} + \frac {\varepsilon} {2M} \cdot M
= \varepsilon
\end{align}$

which means, that both sequences $\left| S_{n} \right|$ and $S_{n}$ converge zero.{{Cite book |last1=Ljashko |first1=Ivan Ivanovich |title=Математический анализ: введение в анализ, производная, интеграл. Справочное пособие по высшей математике. |last2=Bojarchuk |first2=Alexey Klimetjevich |last3=Gaj |first3=Jakov Gavrilovich |last4=Golovach |first4=Grigory Petrovich |date=2001 |publisher=Editorial URSS |isbn=978-5-354-00018-0 |editor-first= |edition=1st |series= |volume=1 |location=Moskva |pages=58 |language=ru |trans-title=Mathematical analysis: the introduction into analysis, derivatives, integrals. The handbook to mathematical analysis. |chapter=}}

= Proving 2. =

$\lim_{n \to \infty} \left( z_{n} - z_{\infty} \right) = 0$ . Applying the already proven statement yields $\lim_{n \to \infty} \sum_{m=1}^{n} \big( a_{n,m} \left( z_{n} - z_{\infty} \right) \big) = 0$ . Finally,

$\lim_{n \to \infty} S_{n}
= \lim_{n \to \infty} \sum_{m=1}^{n} \big( a_{n,m} z_{n} \big)
= \lim_{n \to \infty} \sum_{m=1}^{n} \big( a_{n,m} \left( z_{n} - z_{\infty} \right) \big) + z_{\infty} \lim_{n \to \infty} \sum_{m=1}^{n} \big( a_{n,m} \big)
= 0 + z_{\infty} \cdot 1 = z_{\infty}$ , which completes the proof.

References

=Citations=

=Further reading=

Toeplitz, Otto (1911) "[http://matwbn.icm.edu.pl/ksiazki/pmf/pmf22/pmf2219.pdf Über allgemeine lineare Mittelbildungen.]" Prace mat.-fiz., 22, 113–118 (the original paper in German)
Silverman, Louis Lazarus (1913) "On the definition of the sum of a divergent series." University of Missouri Studies, Math. Series I, 1–96
{{citation|title=Divergent Series|first=G. H.|last=Hardy|authorlink=G. H. Hardy|publication-place=Oxford|publisher=Clarendon Press|year=1949|url=https://archive.org/details/divergentseries033523mbp}}, 43-48.
{{cite book | last= Boos | first = Johann | title = Classical and modern methods in summability | year = 2000| isbn = 019850165X | location = New York | publisher=Oxford University Press | url=https://books.google.com/books?id=kZ9cy6XyidEC}}

Category:Theorems in mathematical analysis

Category:Summability methods

Category:Summability theory