a(n) = position of 2*n^3 in A003325.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =18a(5) =25a(6) =33a(7) =44a(8) =57a(9) =68a(10) =81a(11) =99a(12) =116a(13) =134a(14) =152a(15) =177a(16) =200a(17) =223a(18) =246a(19) =276a(20) =304a(21) =331a(22) =360a(23) =397a(24) =433a(25) =465a(26) =501a(27) =541a(28) =579a(29) =617