This may be a bad time of year for this, but I am hoping there is enough tease in this to catch someone’s attention.

Given a list of numbers, for example, 3, 4, 6, 8 and 9, remove two of the numbers a and b from the list and replace them with a*b + a + b. Keep doing this until one number remains. No matter what order you do this in the answer will be the same, in this case 12,599.

Hint: a*b + a + b can almost be factored.

I will provide the short answer and also use this problem as an introduction to the concept of isomorphism, which tells when two operations are structually identical.