N in mathematics! ! This is called double factorial. But we usually use n! Stands for factorial, so we use m! ! It means double factorial, because the concept of factorial will be used in the expression of double factorial, so it should be represented by different letters. But no matter which letter is used, the meaning is the same.

Double factorial is a mathematical concept, using n! ! Express delivery. The double factorial of a positive integer represents all positive integer products that do not exceed this positive integer and have the same parity with it. The double factorial of the first six positive integers is: 1! ! = 1,2! ! =2,3! ! =3,4! ! =8,5! ! = 15 and 6! ! =48。

Example:

3! ! = 1×3=3

5! ! = 1×3×5= 15

6! ! =2×4×6=48

Extended data:

For complex numbers, it should refer to the product of congruences of all modulo n less than or equal to │ n. The standard expression of any real number n is:

Positive number n = m+x, where m is its positive part and x is its fractional part.

The negative number n =-m -x, where -m is its positive part and -x is its fractional part.

For pure complex numbers, n=(m+x)i, or n =-(m+x) i.

Let's extend factorial to pure complex numbers:

Positive real factorial: n! =│n│！ =n(n- 1)(n-2)....( 1+x)。 x！ =(i^4m).│n│！

Negative real factorial: (-n)! =cos(mπ)│n│！ =(i^2m)..(n- 1)(n-2)....( 1+x)。 x！

References:

Baidu Encyclopedia-Double Factor