The double factorial of 2n is equal to the n power of 2 multiplied by the factorial of n. According to the mathematical algorithm, the double factorial of 2n is equal to the n power of 2 multiplied by the factorial of n, and the second order of 2n contains all odd numbers from 1 to 2n minus 1. The factorial of n includes the product of all integers from 1 to n, because the product of odd numbers includes all even numbers from 1 to n, that is, the double factorial of 2n is equal to the n power of 2 multiplied by the factorial of n..