P and a are the same, they are all arranged, and P is an old usage. Now textbooks often use a to arrange n elements from m elements, that is to say, after taking out n elements, these n elements have to be sorted to get the sorted species number.

C is a combination, that is, only n is taken from m, and the number of species is obtained without sorting.

The relationship between a and c is Amn=Cmn×n! , where n! That is, the number of species arranged in n numbers, which is the difference between them.

There is only one combination of n in CNN =1n.

Pnn=Ann=n！