a(n) = -n^2 + 21*n - 1.

Terms

a(0) =19a(1) =37a(2) =53a(3) =67a(4) =79a(5) =89a(6) =97a(7) =103a(8) =107a(9) =109a(10) =109a(11) =107a(12) =103a(13) =97a(14) =89a(15) =79a(16) =67a(17) =53a(18) =37a(19) =19a(20) =-1a(21) =-23a(22) =-47a(23) =-73a(24) =-101a(25) =-131a(26) =-163a(27) =-197a(28) =-233a(29) =-271