@BreakTheCipher Thanks, that’s an interesting list of challenges. My non-optimal implementation of the Sieve of Eratosthenes still hadn’t found a solution to problem 10 page 5 after several hours, but an implementation of the Sieve of Sundaram found the correct solution after 53 ms. If I get chance I’ll have a look at the method described in your original post. Although I have to admit to feeling a bit of a fraud when using an algorithm that I don’t fully understand. I’ll read the comments on the PE forum to see if that sheds any further light.