13. Gamma Function 1.Definitions, Properties 2.Digamma & Polygamma Functions 3.The Beta Function 4.Sterling’s Series 5.Riemann Zeta Function 6.Other Related Functions
Peculiarities: 1. Do not satisfy any differential equation with rational coefficients. 2. Not a hypergeometric nor a confluent hypergeometric function. Common occurence: In expansion coefficients.
13.1.Definitions, Properties Definition, infinite limit (Euler) version :
Definition: Definite Integral Definition, definite integral (Euler) version : , else singular at t = 0.
Equivalence of the Limit & Integral Definitions Consider
Definition: Infinite Product (Weierstrass Form) Definition, Infinite Product (Weierstrass) version : Euler-Mascheroni constant Proof :
Functional Relations Reflection formula : ( about z = ½ ) Proof : Let
f (z) has pole of order m at z 0 : For z integers, set branch cut ( for v z ) = + x-axis :
Legendre’s Duplication Formula General proof in §13.3. Proof for z = n = 1, 2, 3, …. : ( Case z = 0 is proved by inspection. )
Analytic Properties Weierstrass form : has simple zeros at z n, no poles. (z) has simple poles at z n, no zeros. changes sign at z n. Minimum of for x > 0 is Mathematica
Residues at z n Residue at simple pole z n is n + 1 times :
Schlaefli Integral Schlaefli integral : Proof : C 1 is an open contour. ( e t for Re t . Branch-cut. ) if > 1
For Re < 1, I A, I B, & I D are all singular. However, remains finite. ( integrand regular everywhere on C ) Factorial function : (z) is the Gauss’ notation For Re > 1, I D = 0 reproduces the integral represention. where is valid for all.
Example Maxwell-Boltzmann Distribution Classical statistics (for distinguishable particles) : Probability of state of energy E being occupied is Maxwell-Boltzmann distribution Partition function Average energy : g(E) = density of states Ideal gas : gamma distribution
13.2.Digamma & Polygamma Functions Digamma function : 50 digits z = integer : Mathematica
Polygamma Function Polygamma Function : = Reimann zeta function Mathematica
Maclaurin Expansion of ln Converges for Stirling’s series ( § 13.4 ) has a b etter convergence.
Series Summation Example Catalan’s Constant Dirichlet series : Catalan’s Constant : 20 digits Mathematica
13.3.The Beta Function Beta Function :
Alternate Forms : Definite Integrals To be used in integral rep. of Bessel (Ex ) & hypergeometric (Ex ) functions
Derivation: Legendre Duplication Formula