A026186 a(n) = (1/3)*(s(n) + 2), where s(n) is the n-th number congruent to 1 mod 3 in A026136.

1, 3, 2, 7, 9, 4, 5, 15, 6, 19, 21, 8, 25, 27, 10, 11, 33, 12, 13, 39, 14, 43, 45, 16, 17, 51, 18, 55, 57, 20, 61, 63, 22, 23, 69, 24, 73, 75, 26, 79, 81, 28, 29, 87, 30, 31, 93, 32, 97, 99, 34, 35, 105, 36, 37, 111, 38, 115, 117, 40, 41, 123