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In mathematics, Legendre's formula gives an expression for the exponent of the largest power of a primep that divides the factorialn!. It is named after Adrien-Marie Legendre. It is also sometimes known as de Polignac's formula, after Alphonse de Polignac.
For any prime number p and any positive integer n, let be the exponent of the largest power of p that divides n (that is, the p-adic valuation of n). Then
where is the floor function. While the sum on the right side is an infinite sum, for any particular values of n and p it has only finitely many nonzero terms: for every i large enough that , one has . This reduces the infinite sum above to
Since is the product of the integers 1 through n, we obtain at least one factor of p in for each multiple of p in , of which there are . Each multiple of contributes an additional factor of p, each multiple of contributes yet another factor of p, etc. Adding up the number of these factors gives the infinite sum for .
Legendre's formula can be used to prove Kummer's theorem. As one special case, it can be used to prove that if n is a positive integer then 4 divides if and only if n is not a power of 2.