May 2016 Fiendish Function

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

• GUESS
• REPLY
• 1
• 3
• 2
• 23
• 3
• 17
• 4
• 5
• 5
• 7
• 10
• 337

The rule was: "Write the number followed by the next number, then take the largest prime divisor".

Suppose the guess was 11:

• Write it followed by the next number, giving 1112.
• Find the number's largest prime divisor, giving 139.

So the result is 139.