A symmetrical triangle sequence: T(n, k) = q^k + q^(n-k) - q^n, with q=5.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =-15a(5) =1a(6) =1a(7) =-95a(8) =-95a(9) =1a(10) =1a(11) =-495a(12) =-575a(13) =-495a(14) =1a(15) =1a(16) =-2495a(17) =-2975a(18) =-2975a(19) =-2495a(20) =1a(21) =1a(22) =-12495a(23) =-14975a(24) =-15375a(25) =-14975a(26) =-12495a(27) =1a(28) =1a(29) =-62495