Single exclamation point refers to factorial (all positive integer products less than or equal to this integer)

n！ = 1*2*…………n

The double exclamation mark is an extension of it (when the number is even, it is a positive even product less than or equal to an integer; When the number is odd, it is less than or equal to the positive odd product of the integer)

2n！ ! =2*4*……*2n

(2n+ 1)！ ! = 1*3*……*(2n+ 1)