a(n) = 3*(2*n^2 + 1).

Terms

a(0) =3a(1) =9a(2) =27a(3) =57a(4) =99a(5) =153a(6) =219a(7) =297a(8) =387a(9) =489a(10) =603a(11) =729a(12) =867a(13) =1017a(14) =1179a(15) =1353a(16) =1539a(17) =1737a(18) =1947a(19) =2169a(20) =2403a(21) =2649a(22) =2907a(23) =3177a(24) =3459a(25) =3753a(26) =4059a(27) =4377a(28) =4707a(29) =5049