Double factorial

Double factorial is a method of calculating how many times a number is repeated in a geometric equation. This way, we can calculate the number of times a product is used in its life-time.
The double factorial of "n" is written as formula_1. When "n" is a positive odd integer, formula_1 is defined as formula_3. When n is an positive even integer, formula_1 is defined as formula_5. By definition, formula_6.