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The alternating factorial is the absolute value of the alternating sum of the first factorials, = ()!. These have mainly been studied in connection with their primality; only finitely many of them can be prime, but a complete list of primes of this form is not known.
7.2 Sum of reciprocal of factorials. 7.3 Trigonometry and ... Modified-factorial denominators
Let be a natural number. For a base >, we define the sum of the factorials of the digits [5] [6] of , :, to be the following: = =!. where = ⌊ ⌋ + is the number of digits in the number in base , ! is the factorial of and
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 .
The sum is a sum over all partitions of p. Another exact nested sum expansion for these Stirling numbers is computed by elementary symmetric polynomials corresponding to the coefficients in x {\displaystyle x} of a product of the form ( 1 + c 1 x ) ⋯ ( 1 + c n − 1 x ) {\displaystyle (1+c_{1}x)\cdots (1+c_{n-1}x)} .
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 (!
In the same way that the double factorial generalizes the notion of the single factorial, the following definition of the integer-valued multiple factorial functions (multifactorials), or α-factorial functions, extends the notion of the double factorial function for positive integers : ! = {()!
The sum is taken over all combinations of nonnegative integer indices k 1 through k m such that the sum of all k i is n. That is, for each term in the expansion, the exponents of the x i must add up to n. [2] [a] In the case m = 2, this statement reduces to that of the binomial theorem. [2]