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The gamma function is an important special function in mathematics.Its particular values can be expressed in closed form for integer and half-integer arguments, but no simple expressions are known for the values at rational points in general.
Perhaps the best-known value of the gamma function at a non-integer argument is =, which can be found by setting = in the reflection or duplication formulas, by using the relation to the beta function given below with = =, or simply by making the substitution = in the integral definition of the gamma function, resulting in a Gaussian integral.
Repeated application of the recurrence relation for the lower incomplete gamma function leads to the power series expansion: [2] (,) = = (+) (+) = = (+ +). Given the rapid growth in absolute value of Γ(z + k) when k → ∞, and the fact that the reciprocal of Γ(z) is an entire function, the coefficients in the rightmost sum are well-defined, and locally the sum converges uniformly for all ...
Thus computing the gamma function becomes a matter of evaluating only a small number of elementary functions and multiplying by stored constants. The Lanczos approximation was popularized by Numerical Recipes , according to which computing the gamma function becomes "not much more difficult than other built-in functions that we take for granted ...
This has led to much research and generalization. In particular there is an analog of the Chowla–Selberg formula for p-adic numbers, involving a p-adic gamma function, called the Gross–Koblitz formula. The Chowla–Selberg formula gives a formula for a finite product of values of the eta functions.
The roots of the digamma function are the saddle points of the complex-valued gamma function. Thus they lie all on the real axis. The only one on the positive real axis is the unique minimum of the real-valued gamma function on R + at x 0 = 1.461 632 144 968 362 341 26.... All others occur single between the poles on the negative axis:
It is sometimes referred to as the log-gamma distribution. [20] Formulas for its mean and variance are in the section #Logarithmic expectation and variance. If X ~ Gamma(α, θ), then follows a generalized gamma distribution with parameters p = 2, d = 2α, and = [citation needed].
4.2 Table of values. ... In terms of the regularized gamma function P and the ... Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables ...