A combination in combinatorics is a selection of k elements from n, where order does not matter.
Number of combinations (without repetition)
$$C_{n}^{k}=\frac{n!}{k!(n-k)!}$$
where,
n— number of elements;
k— number of selected elements.
Number of combinations (with repetition)
$$\overline{C}_{n}^{k}=\binom{n+k-1}{k}$$
where,
n— number of elements;
k— number of selected elements.
See also:
Combinations FAQ
What is a combination?
It is an unordered selection of k elements from n.
What is the formula without repetition?
C(n, k) = n! / (k!(n-k)!).
