proof of Gauss’ digamma theorem


Proof. The proof follows the argumentMathworldPlanetmath given in [1], which in turn derives from that given in [2].

The first formula is the logarithmic derivativeMathworldPlanetmath of

Γ(x+n)=(x+n-1)(x+n-2)xΓ(x)

By the partial fraction decomposition satisfied by the ψ functionMathworldPlanetmath,

ψ(pq)+γ=n=0(1n+1-qp+nq)=limt1-n=0(1n+1-qp+nq)tp+nq

using Abel’s limit theorem.

Now,

n=0(1n+1-qp+nq)tp+nq=n=0tp+nqn+1-n=0qtp+nqp+nq=tp-qn=0t(n+1)qn+1-qn=0tp+nqp+nq

Since

-ln(1-t)=n=1tnn

the first term is

-tp-qln(1-tq)

Using the algorithm for extracting every qth term of a series (http://planetmath.org/ExtractingEveryNthTermOfASeries), the second term is

n=0q-1ω-npln(1-ωnt)

and therefore

n=0(1n+1-qp+nq)tp+nq =-tp-qln(1-tq)+n=0q-1ω-npln(1-ωnt)
=-tp-qln1-tq1-t-(tp-1-1)ln(1-t)+n=1q-1ω-npln(1-ωnt)

Let t1- to get

ψ(pq)=-γ-lnq+n=1q-1ω-npln(1-ωn)

Replace p by q-p and add the two expressions to obtain

ψ(pq)+ψ(q-pq)=-2γ-2lnq+2n=1q-1cos(2πnpq)ln(1-ωn)

The left side is real, so it is equal to the real part of the right side. But

(ln(1-ωn))=ln|1-ωn|1/2=ln|(1-cos2πnq)2+sin22πnq|1/2=12ln(2-2cos2πnq)

and so

ψ(pq)+ψ(q-pq)=-2γ-2lnq+n=1q-1cos(2πnpq)ln(2-2cos2πnq) (1)

But

ψ(x)-ψ(1-x)=ddxln(Γ(x)Γ(1-x))=-πcotπx

by the Euler reflection formula and thus

ψ(pq)-ψ(q-pq)=-πcotπpq (2)

Add equations (1) and (2) to get

ψ(pq) =-γ-π2cotπpq-lnq+12n=1q-1cos2πnpqln(2-2cos2πnq)
=-γ-π2cotπpq-lnq+n=1q-1cos2πnpqln(2sinπnq)

where the last equality holds since

ln(2-2cos(2θ))=ln(2-2(1-2sin2θ)=ln(4sin2θ)=2ln(2sinθ)

References

  • 1 G.E. Andrews, R. Askey, R. Roy, Special Functions, Cambridge University Press, 2001.
  • 2 J.L. Jensen [1915-1916], An elementary exposition of the theory of the gamma functionDlmfDlmfMathworldPlanetmath, Ann. Math. 17, 124-166.
Title proof of Gauss’ digamma theorem
Canonical name ProofOfGaussDigammaTheorem
Date of creation 2013-03-22 16:24:07
Last modified on 2013-03-22 16:24:07
Owner rm50 (10146)
Last modified by rm50 (10146)
Numerical id 5
Author rm50 (10146)
Entry type Proof
Classification msc 30D30
Classification msc 33B15