a(n) = (1 - (-4)^n)/5.

Terms

a(0) =1a(1) =-3a(2) =13a(3) =-51a(4) =205a(5) =-819a(6) =3277a(7) =-13107a(8) =52429a(9) =-209715a(10) =838861a(11) =-3355443a(12) =13421773a(13) =-53687091a(14) =214748365a(15) =-858993459a(16) =3435973837a(17) =-13743895347a(18) =54975581389a(19) =-219902325555a(20) =879609302221