A095720 a(1)=1 and, for n>1, a(n)=a(n-1)+Floor(3n/4) if n is even, else a(n)=smallest positive integer which has not yet appeared in the sequence.

1, 3, 2, 5, 4, 8, 6, 12, 7, 15, 9, 18, 10, 20, 11, 23, 13, 27, 14, 29, 16, 32, 17, 35, 19, 39, 21, 42, 22, 44, 24, 48, 25, 51, 26, 53, 28, 56, 30, 60, 31, 63, 33, 66, 34, 68, 36, 72, 37, 75, 38, 77, 40, 80, 41, 83, 43, 87, 45, 90, 46, 92, 47, 95, 49, 99, 50, 101, 52, 104, 54