Combinations Formula
Permutations of n items
Permutations of r items out of n items:
Permutations of n items arranged in a circle
Combinations
C(n,r) - Select r things from n items
Combination with repetition
(Selecting n things from r types of things)
Number of handshakes among n people
Circular Handshakes
(only shake with immediate neighbors on your left and right)
Distribute n identical items into r groups
(Stars And Bars) / Sticks And Stones
(Bowling Pin)/Triangular Numbers
Formula for n-th triangular number