Counts for 2-digit primes Vertical: digit sum (multiples of 3 omitted) Horizontal: first digit | 1 2 3 4 5 6 7 8 9 | total ----+-------------------------------------+------ 2 | 1 | 1 4 | 1 1 | 2 5 | 1 1 | 2 7 | 1 1 | 2 8 | 1 1 1 | 3 10 | 1 1 1 | 3 11 | 1 1 1 | 3 13 | 1 | 1 14 | 1 | 1 16 | 1 1 | 2 17 | 1 | 1 ----+-------------------------------------+------ tot | 4 2 2 3 2 2 3 2 1 | 21 Max count is 3 for digit sum 8