February 2018 Fiendish Function

The Fiendish Function posed during February 2018 had the following sample guesses and replies:

  • GUESS
  • REPLY
  • 1
  • 3
  • 2
  • 9
  • 3
  • 19
  • 4
  • 33
  • 5
  • 51
  • 10
  • 201

The rule was: "Two times the number squared plus one".

Suppose the guess was 12:

  • Take 2 times the number squared, giving 288
  • Add 0 times the number, giving 288
  • Add 1, giving 289

So the result is 289.