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Peter Luschny, Approximation formulas for the factorial function n! Weisstein, Eric W. , "Stirling's Approximation" , MathWorld Stirling's approximation at PlanetMath .
Stirling numbers express coefficients in expansions of falling and rising factorials (also known as the Pochhammer symbol) as polynomials.. That is, the falling factorial, defined as = (+) , is a polynomial in x of degree n whose expansion is
The falling factorial occurs in a formula which represents polynomials using the forward difference operator = (+) , which in form is an exact analogue to Taylor's theorem: Compare the series expansion from umbral calculus
The word "factorial" (originally French: factorielle) was first used in 1800 by Louis François Antoine Arbogast, [18] in the first work on Faà di Bruno's formula, [19] but referring to a more general concept of products of arithmetic progressions. The "factors" that this name refers to are the terms of the product formula for the factorial. [20]
A more efficient method to compute individual binomial coefficients is given by the formula = _! = () (()) () = = +, where the numerator of the first fraction, _, is a falling factorial. This formula is easiest to understand for the combinatorial interpretation of binomial coefficients.
Other extensions of the factorial function do exist, but the gamma function is the most popular and useful. It appears as a factor in various probability-distribution functions and other formulas in the fields of probability, statistics, analytic number theory, and combinatorics.
Here, n! denotes the factorial of n. The function f (n) (a) denotes the n th derivative of f evaluated at the point a. The derivative of order zero of f is defined to be f itself and (x − a) 0 and 0! are both defined to be 1. This series can be written by using sigma notation, as in the right side formula. [1]
These are counted by the double factorial 15 = (6 − 1)‼. In mathematics, the double factorial of a number n, denoted by n‼, is the product of all the positive integers up to n that have the same parity (odd or even) as n. [1] That is,