in Mathematics we use the following definition:

• If the order does matter it is a Permutation.
• If the order doesn't matter, it is a Combination.

Permutation Formula

Suppose 0!=1

p(n,m)=n(n-1)(n-2)……(n-m+1)= n!/(n-m)!

Combination Formula

c(n,m)=p(n,m)/m!=n!/((n-m)!*m!)；c(n,m)=c(n,n-m);