Double factorial
Writing functions:
factorialdbl(x)
Unlike factorial, where the product of the numbers from 1 to the indicated number, then double factorial is a multiplication of numbers from 1 (or 2) to indicated number from step 2. The number x is always finite, for example:
factorialdbl(7)=1*3*5*7
factorialdbl(12)=2*4*6*10*12