a(n) = 28*n^2 - 1.

Terms

a(0) =27a(1) =111a(2) =251a(3) =447a(4) =699a(5) =1007a(6) =1371a(7) =1791a(8) =2267a(9) =2799a(10) =3387a(11) =4031a(12) =4731a(13) =5487a(14) =6299a(15) =7167a(16) =8091a(17) =9071a(18) =10107a(19) =11199a(20) =12347a(21) =13551a(22) =14811a(23) =16127a(24) =17499a(25) =18927a(26) =20411a(27) =21951a(28) =23547a(29) =25199