The unique prime factorisation of 12. 12 factors into 2, 2 and 3. If a column is index 2n or 3n it will never contain a prime after the first row. The columns

The unique prime factorisation of 12. 12 factors into 2, 2 and 3. If a column is index 2n or 3n it will never contain a prime after the first row. The columns that match that here are: 2, 3, 4, 6, 8, 10, 12. The columns that don't are 1, 5, 7, 11 - your four.