Calcudoku puzzle forum

This one's a bit more challenging, but the rules are still simple. The clues hint at both which cells contain primes, and when sums of entries are prime. More info, a solution, and a PDF version at the following link:

https://quadratablog.blogspot.com/2021/ ... udoku.html

This is pretty clever. I haven't given it a lot of thought but it seems to have its own solving strategies.
If a 2-circle implies that one of the cells must be 2 since its the only even prime.
Similarly, a 0-circle implies that one of the cells must be 9 since its the only odd composite number less than 10.

Thanks for the compliment! Yes, there's definitely some new tricks I needed to come up with to make and solve these. One interesting tidbit, is that I couldn't do the same sort of thing for differences at all (without getting tons of extra unwanted solutions.) There's a delicate balance that allows a single solution to sneak out in this sum case.