It's equivalent to http://oeis.org/A054521 http://oeis.org/A054521 Cell i of row j contains n elements of an arithmetic progression, with common difference of

It's equivalent to http://oeis.org/A054521 http://oeis.org/A054521 Cell i of row j contains n elements of an arithmetic progression, with common difference of j. If i and j have a GCD != 1, they are not coprime, so they share a factor p, as do all numbers in the sequence, so they cannot be prime Otherwise it's very likely there's at least one prime If j is prime every column i is coprime with it, so it's going to be black