Extended description:
Definition of 1. Combination Number
Taking out m different elements from n different elements at a time, regardless of the order, is called choosing the combination of m elements from n elements without repetition. The number of species in all such combinations is called the combination number.
2. The nature of combination number
(1) Complementarity: that is, the number of combinations of n elements of m different elements = the number of combinations of (m-n) elements of m different elements;
(2) 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).