a(n) = position of 3*n^3 in A003072.

Terms

a(0) =1a(1) =4a(2) =13a(3) =27a(4) =51a(5) =86a(6) =132a(7) =194a(8) =263a(9) =364a(10) =475a(11) =608a(12) =763a(13) =950a(14) =1166a(15) =1402a(16) =1675a(17) =1976a(18) =2326a(19) =2683a(20) =3114a(21) =3571a(22) =4057a(23) =4579a(24) =5150a(25) =5782a(26) =6441a(27) =7185a(28) =7943a(29) =8797