How to deal with infinitely large sets

[@Prabhu108.]

Hey could you bring some questions of functions?
Great work btw 👍🏻

[@Mather4269mixti]

Nice solution

[@alinpopescu4147]

Recently I solved a problem to find all finite sets that satisfy: if a and b distinct elements of S then (a+b) / gcd(a,b) is also in S. The funny thing is, the solution is:
exactly the same, just characterizing the 3 smallest elements.