Sum_{i=0..n} Sum_{j=0..i} a(j) * a(i-j) = (-3)^n.

Terms

a(0) =1a(1) =-2a(2) =4a(3) =-10a(4) =26a(5) =-70a(6) =192a(7) =-534a(8) =1500a(9) =-4246a(10) =12092a(11) =-34606a(12) =99442a(13) =-286730a(14) =829168a(15) =-2403834a(16) =6984234a(17) =-20331558a(18) =59287740a(19) =-173149662a(20) =506376222a(21) =-1482730098a(22) =4346486256a(23) =-12754363650a(24) =37461564504a(25) =-110125172682a(26) =323990062452a(27) =-953883382354