N is an odd number, then n! ! =n*(n-2)*(n-4)*(n-6)*…* 1

N is an even number, then n! ! =n*(n-2)*(n-4)*(n-6)*…*2

3! ! =3* 1=3

Liu Yulian and Fu Peiren talked about this symbol at the beginning of the first volume of Lecture Notes on Mathematical Analysis.