Lindemann–Weierstrass theorem
{{short description|On algebraic independence of exponentials of linearly independent algebraic numbers over Q}}
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{{stack|{{Pi box}}|{{E (mathematical constant)}}}}
In transcendental number theory, the Lindemann–Weierstrass theorem is a result that is very useful in establishing the transcendence of numbers. It states the following: {{math_theorem|name=Lindemann–Weierstrass theorem|if {{math|α1, ..., αn}} are algebraic numbers that are linearly independent over the rational numbers , then {{math|eα1, ..., eαn}} are algebraically independent over .}}
In other words, the extension field has transcendence degree {{math|n}} over .
An equivalent formulation from {{Harvnb|Baker|1990|loc=Chapter 1, Theorem 1.4}}, is the following: {{math_theorem|name=An equivalent formulation|If {{math| α1, ..., αn }} are distinct algebraic numbers, then the exponentials {{math|eα1, ..., eαn}} are linearly independent over the algebraic numbers.}} This equivalence transforms a linear relation over the algebraic numbers into an algebraic relation over by using the fact that a symmetric polynomial whose arguments are all conjugates of one another gives a rational number.
The theorem is named for Ferdinand von Lindemann and Karl Weierstrass. Lindemann proved in 1882 that {{math|eα}} is transcendental for every non-zero algebraic number {{math|α,}} thereby establishing that {{pi}} is transcendental (see below). Weierstrass proved the above more general statement in 1885.
The theorem, along with the Gelfond–Schneider theorem, is extended by Baker's theorem,{{Harvnb|Murty|Rath|2014}} and all of these would be further generalized by Schanuel's conjecture.
Naming convention
The theorem is also known variously as the Hermite–Lindemann theorem and the Hermite–Lindemann–Weierstrass theorem. Charles Hermite first proved the simpler theorem where the {{math|αi}} exponents are required to be rational integers and linear independence is only assured over the rational integers,{{Harvnb|Hermite|1873|pp=18–24}}.{{Harvnb|Hermite|1874}} a result sometimes referred to as Hermite's theorem.{{Harvnb|Gelfond|2015}}. Although that appears to be a special case of the above theorem, the general result can be reduced to this simpler case. Lindemann was the first to allow algebraic numbers into Hermite's work in 1882.{{Harvnb|Lindemann|1882a}}, {{Harvnb|Lindemann|1882b}}. Shortly afterwards Weierstrass obtained the full result,{{Harvnb|Weierstrass|1885|pp=1067–1086}}, and further simplifications have been made by several mathematicians, most notably by David Hilbert{{Harvnb|Hilbert|1893|pp=216–219}}. and Paul Gordan.{{Harvnb|Gordan|1893|pp=222–224}}.
{{anchor|Transcendence of ''e'' and π}} Transcendence of {{math| ''e'' }} and {{pi}}
{{also|e (mathematical constant)|Pi}}
The transcendence of {{math| e }} and {{pi}} are direct corollaries of this theorem.
Suppose {{math| α }} is a non-zero algebraic number; then {{math| {α} }} is a linearly independent set over the rationals, and therefore by the first formulation of the theorem {{math| {eα} }} is an algebraically independent set; or in other words {{math| eα }} is transcendental. In particular, {{math| e1 {{=}} e }} is transcendental. (A more elementary proof that {{math| e }} is transcendental is outlined in the article on transcendental numbers.)
Alternatively, by the second formulation of the theorem, if {{math| α }} is a non-zero algebraic number, then {{math| {0, α} }} is a set of distinct algebraic numbers, and so the set {{math| {e0, eα} {{=}} {1, eα} }} is linearly independent over the algebraic numbers and in particular {{math| eα }} cannot be algebraic and so it is transcendental.
To prove that {{pi}} is transcendental, we prove that it is not algebraic. If {{pi}} were algebraic, {{pi}}i would be algebraic as well, and then by the Lindemann–Weierstrass theorem {{math| e{{pi}}i {{=}} −1 }} (see Euler's identity) would be transcendental, a contradiction. Therefore {{pi}} is not algebraic, which means that it is transcendental.
A slight variant on the same proof will show that if {{math| α }} is a non-zero algebraic number then {{math| sin(α), cos(α), tan(α) }} and their hyperbolic counterparts are also transcendental.
{{anchor|''p''-adic conjecture}} {{math|''p''}}-adic conjecture
{{math_theorem|name={{math|p}}-adic Lindemann–Weierstrass Conjecture.|math_statement=Suppose {{math| p }} is some prime number and {{math| α1, ..., αn }} are p-adic numbers which are algebraic and linearly independent over , such that {{math| {{!}} αi {{!}}p < 1/p }} for all {{math| i; }} then the p-adic exponential functions {{math| expp(α1), . . . , expp(αn) }} are {{math|p}}-adic numbers that are algebraically independent over . }}
Modular conjecture
An analogue of the theorem involving the modular function {{math| j }} was conjectured by Daniel Bertrand in 1997, and remains an open problem.{{Harvnb|Bertrand|1997|pp=339–350}}. Writing {{math| q {{=}} e2{{pi}}iτ }} for the square of the nome and {{math| j(τ) {{=}} J(q), }} the conjecture is as follows. {{math_theorem|name=Modular conjecture|Let {{math| q1, ..., qn }} be non-zero algebraic numbers in the complex unit disc such that the {{math| 3n }} numbers
:
are algebraically dependent over . Then there exist two indices {{math| 1 ≤ i < j ≤ n }} such that {{math| qi }} and {{math| qj }} are multiplicatively dependent.}}
Lindemann–Weierstrass theorem
{{math_theorem|name=Lindemann–Weierstrass Theorem (Baker's reformulation).|math_statement=If {{math| a1, ..., an }} are algebraic numbers, not all zero, and {{math| α1, ..., αn }} are distinct algebraic numbers, then{{citation
| last = Baker | first = Alan
| doi = 10.1017/CBO9781139093835
| isbn = 978-1-107-60379-0
| mr = 2954465
| page = 53
| publisher = Cambridge University Press, Cambridge
| title = A Comprehensive Course in Number Theory
| year = 2012}}
:}}
=Proof=
The proof relies on two preliminary lemmas. Notice that Lemma B itself is already sufficient to deduce the original statement of Lindemann–Weierstrass theorem.
==Preliminary lemmas==
{{math_theorem|name=Lemma A.|math_statement=Let {{math|c(1), ..., c(r)}} be integers and, for every {{mvar|k}} between {{math|1}} and {{mvar|r}}, let {{math|{γ(k)1, ..., γ(k)m(k)} }} be the roots of a non-zero polynomial with integer coefficients . If {{math|γ(k)i ≠ γ(u)v }} whenever {{math|(k, i) ≠ (u, v)}}, then
:
has only the trivial solution for all }}
Proof of Lemma A. To simplify the notation set:
:
\begin{align}
& n_0 =0, & & \\
& n_i =\sum\nolimits_{k=1}^i m(k), & & i=1,\ldots,r \\
& n=n_r, & & \\
& \alpha_{n_{i-1}+j} =\gamma(i)_j, & & 1\leq i\leq r,\ 1\leq j\leq m(i) \\
& \beta_{n_{i-1}+j} =c(i).
\end{align}
Then the statement becomes
:
Let {{mvar|p}} be a prime number and define the following polynomials:
:
where {{mvar|ℓ}} is a non-zero integer such that are all algebraic integers. DefineUp to a factor, this is the same integral appearing in Transcendental number#A proof that e is transcendental, where {{math|β1 {{=}} 1, ..., βm {{=}} m.}} The rest of the proof of the Lemma is analog to that proof.
:
Using integration by parts we arrive at
:
where is the degree of , and is the j-th derivative of . This also holds for s complex (in this case the integral has to be intended as a contour integral, for example along the straight segment from 0 to s) because
:
is a primitive of .
Consider the following sum:
:
J_i &=\sum_{k=1}^n\beta_k I_i(\alpha_k)\\[5pt]
&= \sum_{k=1}^n\beta_k \left ( e^{\alpha_k} \sum_{j=0}^{np-1} f_i^{(j)}(0) - \sum_{j=0}^{np-1} f_i^{(j)}(\alpha_k)\right ) \\[5pt]
&=\left(\sum_{j=0}^{np-1}f_i^{(j)}(0)\right)\left(\sum_{k=1}^n \beta_k e^{\alpha_k}\right)-\sum_{k=1}^n\sum_{j=0}^{np-1} \beta_kf_i^{(j)}(\alpha_k)\\[5pt]
&= -\sum_{k=1}^n \sum_{j=0}^{np-1} \beta_kf_i^{(j)}(\alpha_k)
\end{align}
In the last line we assumed that the conclusion of the Lemma is false. In order to complete the proof we need to reach a contradiction. We will do so by estimating in two different ways.
First is an algebraic integer which is divisible by p! for and vanishes for
==Final step==
We turn now to prove the theorem: Let a(1), ..., a(n) be non-zero algebraic numbers, and α(1), ..., α(n) distinct algebraic numbers. Then let us assume that:
:
We will show that this leads to contradiction and thus prove the theorem. The proof is very similar to that of Lemma B, except that this time the choices are made over the a(i)'s:
For every i ∈ {1, ..., n}, a(i) is algebraic, so it is a root of an irreducible polynomial with integer coefficients of degree d(i). Let us denote the distinct roots of this polynomial a(i)1, ..., a(i)d(i), with a(i)1 = a(i).
Let S be the functions σ which choose one element from each of the sequences (1, ..., d(1)), (1, ..., d(2)), ..., (1, ..., d(n)), so that for every 1 ≤ i ≤ n, σ(i) is an integer between 1 and d(i). We form the polynomial in the variables
:
Since the product is over all the possible choice functions σ, Q is symmetric in
The evaluated polynomial
:
where we already grouped the terms with the same exponent. So in the left-hand side we have distinct values β(1), ..., β(N), each of which is still algebraic (being a sum of algebraic numbers) and coefficients
The sum is nontrivial: if
By multiplying the equation with an appropriate integer factor, we get an identical equation except that now b(1), ..., b(N) are all integers. Therefore, according to Lemma B, the equality cannot hold, and we are led to a contradiction which completes the proof. ∎
Note that Lemma A is sufficient to prove that e is irrational, since otherwise we may write e = p / q, where both p and q are non-zero integers, but by Lemma A we would have qe − p ≠ 0, which is a contradiction. Lemma A also suffices to prove that {{pi}} is irrational, since otherwise we may write {{pi}} = k / n, where both k and n are integers) and then ±i{{pi}} are the roots of n2x2 + k2 = 0; thus 2 − 1 − 1 = 2e0 + ei{{pi}} + e−i{{pi}} ≠ 0; but this is false.
Similarly, Lemma B is sufficient to prove that e is transcendental, since Lemma B says that if a0, ..., an are integers not all of which are zero, then
:
Lemma B also suffices to prove that {{pi}} is transcendental, since otherwise we would have 1 + ei{{pi}} ≠ 0.
==Equivalence of the two statements==
Baker's formulation of the theorem clearly implies the first formulation. Indeed, if
:
is a polynomial with rational coefficients, then we have
:
and since
Now assume that the first formulation of the theorem holds. For
:
As seen in the previous section, and with the same notation used there, the value of the polynomial
:
at
:
has an expression of the form
:
where we have grouped the exponentials having the same exponent. Here, as proved above,
:
\beta(m) = q_{m,1} \alpha(i_1) + \cdots + q_{m,k} \alpha(i_k), && q_{m,j} = \frac{c_{m,j}}{d_{m,j}}; \qquad c_{m,j}, d_{m,j} \in \Z.
\end{align}
For each
:
by
See also
- Gelfond–Schneider theorem
- Baker's theorem; an extension of Gelfond–Schneider theorem
- Schanuel's conjecture; if proven, it would imply both the Gelfond–Schneider theorem and the Lindemann–Weierstrass theorem
Notes
{{Reflist|2}}
References
- {{Citation | last1=Baker | first1=Alan |author-link=Alan Baker (mathematician) | title=Transcendental number theory | url={{Google books|SmsCqiQMvvgC|Transcendental number theory|plainurl=yes}} | publisher=Cambridge University Press | edition=2nd | series=Cambridge Mathematical Library | isbn=978-0-521-39791-9 | mr=0422171 | year=1990}}
- {{Citation |last=Bertrand |first=D. |author-link=Daniel Bertrand |year=1997 |title=Theta functions and transcendence |journal=The Ramanujan Journal |volume=1 |issue=4 |pages=339–350 |doi=10.1023/A:1009749608672 |s2cid=118628723 }}
- {{Citation | last=Gelfond | first=A.O. | author-link=Alexander Gelfond | translator-last=Boron | translator-first=Leo F. | translator-link=Leo F. Boron | orig-year=1960 | year=2015 | title=Transcendental and Algebraic Numbers | publisher=Dover Publications |location=New York |series=Dover Books on Mathematics | isbn=978-0-486-49526-2 |mr=0057921 | url={{Google books|408wBgAAQBAJ|Transcendental and Algebraic Numbers|plainurl=yes}} }}
- {{Citation | last=Gordan | first=P. | author-link=Paul Gordan | year=1893 | title=Transcendenz von {{math|e}} und {{math|π}}. | journal=Mathematische Annalen | volume=43 | issue=2–3 | pages=222–224 | url=https://gdz.sub.uni-goettingen.de/dms/load/img/?PID=GDZPPN002254557&physid=PHYS_0223 | doi=10.1007/bf01443647| s2cid=123203471 }}
- {{Citation | last=Hermite | first=C. | author-link=Charles Hermite | year=1873 | title=Sur la fonction exponentielle. | journal=Comptes rendus de l'Académie des Sciences de Paris | volume=77 | pages=18–24 | url=http://gallica.bnf.fr/ark:/12148/bpt6k3034n/f18.image}}
- {{Citation | last=Hermite | first=C. | author-link=Charles Hermite | year=1874 | title=Sur la fonction exponentielle. | publisher=Gauthier-Villars | place=Paris | url=https://archive.org/details/surlafonctionexp00hermuoft}}
- {{Citation | last=Hilbert | first=D. | author-link=David Hilbert | year=1893 | title=Ueber die Transcendenz der Zahlen {{math|e}} und {{math|π}}. | journal=Mathematische Annalen | volume=43 | issue=2–3 | pages=216–219 | url=https://gdz.sub.uni-goettingen.de/index.php?id=11&PID=GDZPPN002254565 | doi=10.1007/bf01443645 | s2cid=179177945 | access-date=2018-12-24 | archive-url=https://web.archive.org/web/20171006113711/https://gdz.sub.uni-goettingen.de/index.php?id=11&PID=GDZPPN002254565 | archive-date=2017-10-06 | url-status=dead }}
- {{Citation | last=Lindemann | first=F. | author-link=Ferdinand Lindemann | year=1882 | title=Über die Ludolph'sche Zahl. | journal=Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin | volume=2 | pages=679–682 | url=https://archive.org/details/sitzungsberichte1882deutsch/page/679 |ref={{Harvid|Lindemann|1882a}}}}
- {{Citation | last=Lindemann | first=F. | author-link=Ferdinand Lindemann | year=1882 | title=Über die Zahl {{math|π}}. | journal=Mathematische Annalen | volume=20 | issue=2 | pages=213–225 | url=https://gdz.sub.uni-goettingen.de/id/PPN235181684_0020?tify=%7B%22view%22:%22info%22,%22pages%22:%5B227%5D%7D | ref={{Harvid|Lindemann|1882b}} | doi=10.1007/bf01446522 | s2cid=120469397 | access-date=2018-12-24 | archive-url=https://web.archive.org/web/20171006120026/https://gdz.sub.uni-goettingen.de/index.php?id=11&PID=GDZPPN002246910 | archive-date=2017-10-06 | url-status=dead }}
- {{cite book |doi=10.1007/978-1-4939-0832-5_19|chapter=Baker's Theorem |title=Transcendental Numbers |year=2014 |last1=Murty |first1=M. Ram |last2=Rath |first2=Purusottam |pages=95–100 |isbn=978-1-4939-0831-8|url={{Google books|-4jkAwAAQBAJ|page=95|plainurl=yes}}}}
- {{Citation | last=Weierstrass | first=K. | author-link=Karl Weierstrass | year=1885 | title=Zu Lindemann's Abhandlung. "Über die Ludolph'sche Zahl". | journal=Sitzungsberichte der Königlich Preussischen Akademie der Wissen-schaften zu Berlin | volume=5 | pages=1067–1085 | url=https://books.google.com/books?id=jhlEAQAAMAAJ&pg=PA1067}}
Further reading
- {{Citation | last1=Jacobson | first1=Nathan |author-link=Nathan Jacobson | title=Basic Algebra | volume=I | url={{Google books|JHFpv0tKiBAC&printsec|Basic Algebra I|plainurl=yes}} | publisher=Dover Publications | edition=2nd | isbn=978-0-486-47189-1 | orig-year=1985 | year=2009 }}
External links
- {{MathWorld|title=Hermite-Lindemann Theorem|urlname=Hermite-LindemannTheorem}}
- {{MathWorld|title=Lindemann-Weierstrass Theorem|urlname=Lindemann-WeierstrassTheorem}}
{{DEFAULTSORT:Lindemann-Weierstrass theorem}}
Category:Articles containing proofs
Category:E (mathematical constant)