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单词 ProofOfMultiplicationFormulaForGammaFunction
释义

proof of multiplication formula for gamma function


Define the functionMathworldPlanetmath f as

f(z)=nnzk=0n-1Γ(z+kn)Γ(nz)

By the functional equation of the gamma functionDlmfDlmfMathworldPlanetmath,

f(z+1)=nnnnz(m=0n-1Γ(z+mn))k=0n-1(z+kn)Γ(nz)k=0n-1(nz+k)=f(z)

Hence f is a periodic functionMathworldPlanetmath of z. However, for large valuesof z, we can apply the Stirling approximation formula to conclude

f(z)=(2π)n/2nnzk=0n-1[e-z-k/n(z+k/n)z+k/n-1/2+O(e-z(z+k/n)z+k/n-3/2)](2π)1/2e-nz(nz)nz-1/2+O(e-nz(nz)nz-3/2)=
(2π)(n-1)/2n1/2k=0n-1[e-k/n(z+k/n)z+k/n-1/2+O((z+k/n)z+k/n-3/2)]znz-1/2+O(znz-3/2)=
(2π)(n-1)/2n1/2z1/2k=0n-1[e-k/n(1+knz)z+k/n-1/2zk/n-1/2+O((z+k/n)k/n-3/2)]1+O(z-1)

Note that

k=0n-1e-k/n=e-k=0n-1k/n=e(1-n)/2
z1/2k=0n-1zk/n-1/2=z1/2zk=0n-1(k/n-1/2)=z1/2+(n-1)/2-n/2=1

Also,

(1+knz)z=ek/n+O(z-1)

Hence, f(z)=(2π)(n-1)/2n1/2+O(z-1). Now, the only way for a function to be periodic and have a definite limit is for that function to be constant. Therefore, f(z)=(2π)(n-1)/2n1/2. Writing out the definition of f and rearranging gives the multiplication formula.

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更新时间:2025/5/4 4:55:09