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Stirling's approximation. Comparison of Stirling's approximation with the factorial. In mathematics, Stirling's approximation (or Stirling's formula) is an asymptotic approximation for factorials. It is a good approximation, leading to accurate results even for small values of .
n ! {\displaystyle n!} In mathematics, the factorial of a non-negative integer , denoted by , is the product of all positive integers less than or equal to . The factorial of also equals the product of with the next smaller factorial: For example, The value of 0! is 1, according to the convention for an empty product.
The gamma function is the unique function that simultaneously satisfies. , for all complex numbers except the non-positive integers, and, for integer n, for all complex numbers . [1] In a certain sense, the log-gamma function is the more natural form; it makes some intrinsic attributes of the function clearer.
An algorithm is said to be factorial time if T(n) is upper bounded by the factorial function n!. Factorial time is a subset of exponential time (EXP) because n ! ≤ n n = 2 n log n = O ( 2 n 1 + ϵ ) {\displaystyle n!\leq n^{n}=2^{n\log n}=O\left(2^{n^{1+\epsilon }}\right)} for all ϵ > 0 {\displaystyle \epsilon >0} .
In this article, the symbol () is used to represent the falling factorial, and the symbol () is used for the rising factorial. These conventions are used in combinatorics , [ 4 ] although Knuth 's underline and overline notations x n _ {\displaystyle x^{\underline {n}}} and x n ¯ {\displaystyle x^{\overline {n}}} are increasingly popular.
In mathematics, the logarithm to base b is the inverse function of exponentiation with base b. That means that the logarithm of a number x to the base b is the exponent to which b must be raised to produce x. For example, since 1000 = 103, the logarithm base of 1000 is 3, or log10 (1000) = 3.
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]
The natural logarithm of x is generally written as ln x, loge x, or sometimes, if the base e is implicit, simply log x. [2][3] Parentheses are sometimes added for clarity, giving ln (x), loge(x), or log (x). This is done particularly when the argument to the logarithm is not a single symbol, so as to prevent ambiguity.