a(n) = (2*4^n + 7)/3.

Terms

a(0) =3a(1) =5a(2) =13a(3) =45a(4) =173a(5) =685a(6) =2733a(7) =10925a(8) =43693a(9) =174765a(10) =699053a(11) =2796205a(12) =11184813a(13) =44739245a(14) =178956973a(15) =715827885a(16) =2863311533a(17) =11453246125a(18) =45812984493a(19) =183251937965a(20) =733007751853