It's fine to work on any problem, so long as it generates interesting mathematics along the way - even if you don't solve it at the end of the day.
Andrew WilesI know it's a rare privilege, but if one can really tackle something in adult life that means that much to you, then it's more rewarding than anything I can imagine.
Andrew WilesWe've lost something that's been with us for so long, and something that drew a lot of us into mathematics. But perhaps that's always the way with math problems, and we just have to find new ones to capture our attention.
Andrew Wiles