A076972 a(n) = index of the triangular number A076971(n).

1, 2, 3, 5, 7, 10, 15, 21, 30, 43, 61, 87, 123, 174, 247, 349, 494, 699, 989, 1399, 1979, 2799, 3959, 5599, 7919, 11199, 15838, 22399, 31677, 44799, 63355, 89598, 126711, 179197, 253423, 358395, 506847, 716790, 1013695, 1433581, 2027390, 2867163