Permutation and Combination Formula Factorial Notation Let n be a positive integer. Then, factorial n, denoted n! is defined as: n! = n(n – 1)(n – 2) … 3.2.1. POINTS TO REMEMBER 0! = 1. 1! = 1. 2! = 2. 3! = 6. 4! = 24. 5! = 120….