a(n) = (6-(-4)^n)/5.

Terms

a(0) =1a(1) =2a(2) =-2a(3) =14a(4) =-50a(5) =206a(6) =-818a(7) =3278a(8) =-13106a(9) =52430a(10) =-209714a(11) =838862a(12) =-3355442a(13) =13421774a(14) =-53687090a(15) =214748366a(16) =-858993458a(17) =3435973838a(18) =-13743895346a(19) =54975581390a(20) =-219902325554a(21) =879609302222