Even octagonal numbers: a(n) = 4*n*(3*n-1).

Terms

a(0) =0a(1) =8a(2) =40a(3) =96a(4) =176a(5) =280a(6) =408a(7) =560a(8) =736a(9) =936a(10) =1160a(11) =1408a(12) =1680a(13) =1976a(14) =2296a(15) =2640a(16) =3008a(17) =3400a(18) =3816a(19) =4256a(20) =4720a(21) =5208a(22) =5720a(23) =6256a(24) =6816a(25) =7400a(26) =8008a(27) =8640a(28) =9296a(29) =9976