A026259 a(n) = (1/2)*s(n), where s(n) is the n-th even number in A026255.

1, 2, 3, 7, 4, 8, 9, 5, 6, 13, 14, 15, 10, 20, 11, 21, 22, 12, 26, 27, 28, 16, 17, 33, 18, 34, 35, 19, 39, 40, 41, 23, 24, 46, 25, 47, 52, 53, 54, 29, 30, 31, 59, 32, 60, 65, 66, 67, 36, 37, 38, 72, 73, 78, 42, 79, 80, 43, 44, 45, 85, 86, 48