A symmetrical triangle sequence: T(n, k) = q^k + q^(n-k) - q^n, with q=3.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =-3a(5) =1a(6) =1a(7) =-15a(8) =-15a(9) =1a(10) =1a(11) =-51a(12) =-63a(13) =-51a(14) =1a(15) =1a(16) =-159a(17) =-207a(18) =-207a(19) =-159a(20) =1a(21) =1a(22) =-483a(23) =-639a(24) =-675a(25) =-639a(26) =-483a(27) =1a(28) =1a(29) =-1455