(4^(n+1)-(-1)^n 9 )/5.

Terms

a(0) =5a(1) =11a(2) =53a(3) =203a(4) =821a(5) =3275a(6) =13109a(7) =52427a(8) =209717a(9) =838859a(10) =3355445a(11) =13421771a(12) =53687093a(13) =214748363a(14) =858993461a(15) =3435973835a(16) =13743895349a(17) =54975581387a(18) =219902325557a(19) =879609302219