This indicates the combination number. From N different elements, any m(m≤n) elements are grouped, which is called the combination of M elements of N different elements. The number of all combinations of m(m≤n) elements from n different elements is called the number of combinations of m elements from n different elements.

Complementary attribute

That is, the number of combinations of m elements from n different elements = the number of combinations of (n-m) elements from n different elements.

This property is easy to understand, such as c (9,2 2) = c (9 9,7), that is, the method of selecting two elements from nine elements is equal to the method of selecting seven elements from nine elements. Provisions: C(n, 0)= 1 C(n, n) =1c (0,0)= 1.

Combinatorial identity: if m term is selected from n terms, the following formula exists: C(n, m)=C(n, n-m)=C(n- 1, m- 1)+C(n- 1, m).