a(n) = 22*n^2 - 1.

Terms

a(0) =21a(1) =87a(2) =197a(3) =351a(4) =549a(5) =791a(6) =1077a(7) =1407a(8) =1781a(9) =2199a(10) =2661a(11) =3167a(12) =3717a(13) =4311a(14) =4949a(15) =5631a(16) =6357a(17) =7127a(18) =7941a(19) =8799a(20) =9701a(21) =10647a(22) =11637a(23) =12671a(24) =13749a(25) =14871a(26) =16037a(27) =17247a(28) =18501a(29) =19799