Permutations and Combinations – Notes
Fundamental Principle of Counting
If an event can occur in m different ways, following which another event can occur in n different ways, then the total number of the occurrence of the events in the given order is m n.
Introduction to Permutation
A permutation is an arrangement in a definite order of a number of objects taken some or all at a time.
The number of permutations of n different objects taken r at a time is equal to nPr, where 0 < r n.
The number of permutations of n distinct objects taken r at a time, where repetition of the objects is allowed, is nr.