A026253 a(n) = (1/2)*s(n), where s(n) is the n-th even number in A026250. Also a(n) = position of n in A026253.

3, 5, 1, 10, 2, 15, 17, 20, 22, 4, 27, 29, 32, 34, 6, 39, 7, 44, 46, 8, 51, 9, 56, 58, 61, 63, 11, 68, 12, 73, 75, 13, 80, 14, 85, 87, 90, 92, 16, 97, 99, 102, 104, 18, 109, 19, 114, 116, 119, 121, 21, 126, 128, 131, 133, 23, 138, 24, 143, 145