a(n) = ceiling( (3*n^2 - 4)/8 ).

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =6a(5) =9a(6) =13a(7) =18a(8) =24a(9) =30a(10) =37a(11) =45a(12) =54a(13) =63a(14) =73a(15) =84a(16) =96a(17) =108a(18) =121a(19) =135a(20) =150a(21) =165a(22) =181a(23) =198a(24) =216a(25) =234a(26) =253a(27) =273a(28) =294a(29) =315