July 2016 Fiendish Function

The Fiendish Function posed during July 2016 had the following sample guesses and replies:

  • GUESS
  • REPLY
  • 1
  • -1
  • 2
  • -2
  • 3
  • -2
  • 4
  • 8
  • 5
  • 88
  • 10
  • 3627776

The rule was: "Take the factorial of the number, then subtract two to the power of the number".

Suppose the guess was 13:

  • Find 13 x 12 x 11 x 10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1, giving 6227020800.
  • Calculate two to the power of the number, giving 8192.
  • Subtract this, giving 6227012608.

So the result is 6227012608.