A075077 a(1) =1 and then the smallest number not occurring earlier such that a(n)+a(n+1) gives a different composite number (not obtained earlier).

1, 3, 5, 4, 2, 8, 6, 9, 7, 11, 10, 12, 13, 14, 16, 17, 15, 19, 20, 18, 22, 23, 21, 25, 24, 26, 28, 27, 29, 31, 32, 30, 34, 35, 33, 37, 38, 36, 40, 41, 39, 43, 42, 44, 46, 45, 47, 48, 50, 49, 51, 53, 52, 54, 56, 55, 57, 58, 59, 60, 61, 62, 63, 65, 64, 66, 67, 68, 70, 71, 69, 73