a(0) = 0, a(1) = 1; for n>0, a(2*n) = 3*a(2*n-1), a(2*n+1) = 3*a(2*n) - 2*a(n-1).

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =27a(5) =79a(6) =237a(7) =705a(8) =2115a(9) =6327a(10) =18981a(11) =56889a(12) =170667a(13) =511843a(14) =1535529a(15) =4606113a(16) =13818339a(17) =41453607a(18) =124360821a(19) =373078233a(20) =1119234699a(21) =3357691443a(22) =10073074329a(23) =30219185025a(24) =90657555075a(25) =271972551447a(26) =815917654341