Factorial:  A factorial, denoted by n!, is the product of the consecutive natural integers from 1 to n. Note: 0! = 1 by definition.
  Double factorial: A double factorial, denoted by n!!, is defined for positive even integers as: n * (n - 2) * (n - 4) * ... * 6 * 4 * 2
         and for positive odd integers as: n * (n - 2) * (n - 4) * ... * 5 * 3 * 1. Defined recursively: n!! = n * (n - 2)!! Note: 0!! = 1, -1!!= 1 by definition.
  Triple factorial: A triple factorial, denoted by n!!!, is defined for positive integers as: n * (n - 3) * (n - 6) * ... . Defined recursively: n!!! = n * (n - 3)!!!
          By definition 1!!!= 1, 0!!! = 1, -1!!! = 1
  Primorial:  A primorial, denoted n#, is the product of the first n prime numbers.

  n =

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