k factorial literally means k times k minus 1 times k minus 2, all the way down to 1. For example, 2 factorial is equal to 2 times 1. 3 factorial is equal to 3 times 2 times 1. 4 factorial is equal to 4 times 3 times 2 times 1. It's actually a fun thing to play with. Factorials get large very, very, very, very fast. Anyway, this denominator right over here can be rewritten as k factorial.