In general I think there are a lot of project Euler problems (at least after the first 100) that are basically “here is some theorem/construction you might see

In general I think there are a lot of project Euler problems (at least after the first 100) that are basically “here is some theorem/construction you might see in undergrad (or maybe early postgrad) number theory, except we don’t tell you what it is, instead we just give you a problem to which it applies.” And if you don’t know the theorem you are SOL. When I was last doing PE, that Knowledge was relatively fresh in my mind so I could know that the solution was Pell’s equation or whatever (good luck finding the wiki page if you don’t know the name…) but even then it felt a bit silly. I think the solution I was most proud of was to an older problem (they weren’t adding new ones at the time) with less than 100 solves and it basically boiled down to Gaussian elimination without much mathematical knowledge. Mostly I was proud because I got the achievement for solving one early.