1,4,5,8,9 () 13,16, () how to find the law

1,4,5,8,9 () 13,16, () how to find the law

Group 1: 1 + 3 = 4 group 2: 4 + 1 = 5 group 3: 5 + 3 = 8 group 4: 8 + 1 = 9 group 5: 9 + 3 = 12 group 6: 12 + 1 = 13 group 7: 13 + 3 = 16 group 8: 16 + 1 = 17 rule: odd number + 3; even number + 1