a(n) = 13 + 12*n - n^2.

Terms

a(0) =13a(1) =24a(2) =33a(3) =40a(4) =45a(5) =48a(6) =49a(7) =48a(8) =45a(9) =40a(10) =33a(11) =24a(12) =13a(13) =0a(14) =-15a(15) =-32a(16) =-51a(17) =-72a(18) =-95a(19) =-120a(20) =-147a(21) =-176a(22) =-207a(23) =-240a(24) =-275a(25) =-312a(26) =-351a(27) =-392a(28) =-435a(29) =-480