a(n) = (1 - (-5)^n)/6.

Terms

a(0) =1a(1) =-4a(2) =21a(3) =-104a(4) =521a(5) =-2604a(6) =13021a(7) =-65104a(8) =325521a(9) =-1627604a(10) =8138021a(11) =-40690104a(12) =203450521a(13) =-1017252604a(14) =5086263021a(15) =-25431315104a(16) =127156575521a(17) =-635782877604