What value is assigned to zero factorial?

1. Factorials of positive integers multiply every integer from 1 up to n. 2. Zero factorial is a special case that is defined rather than computed, and its value is 1. 3. That convention is what makes the permutation count of n objects taken n at a time come out as n! divided by 1. 4. If zero factorial were 0 the permutation formula would divide by zero, so leaving it undefined or setting it to zero both break the arithmetic. _Source: OpenStax Algebra and Trigonometry (CC BY 4.0), Ch 13 "Sequences, Probability and Counting Theory", section 13.5 Counting Principles_