1 odd number, 2 even numbers, 3 odd numbers, ...

1, 2, 4, 5, 7, 9, 10, 12, 14, 16, 17, 19, 21, 23, 25, 26, 28, 30, 32, 34, 36, 37, 39, ...

a_n = 2 n - floor(1/2 (sqrt(8 n - 7) + 1))