Concentric hexagonal numbers: floor(3*n^2/2).

Terms

a(0) =0a(1) =1a(2) =6a(3) =13a(4) =24a(5) =37a(6) =54a(7) =73a(8) =96a(9) =121a(10) =150a(11) =181a(12) =216a(13) =253a(14) =294a(15) =337a(16) =384a(17) =433a(18) =486a(19) =541a(20) =600a(21) =661a(22) =726a(23) =793a(24) =864a(25) =937a(26) =1014a(27) =1093a(28) =1176a(29) =1261