a(n) = (2^(2*n + 1) + 1)/3.

Terms

a(0) =1a(1) =3a(2) =11a(3) =43a(4) =171a(5) =683a(6) =2731a(7) =10923a(8) =43691a(9) =174763a(10) =699051a(11) =2796203a(12) =11184811a(13) =44739243a(14) =178956971a(15) =715827883a(16) =2863311531a(17) =11453246123a(18) =45812984491a(19) =183251937963a(20) =733007751851