a(2*n) = 3*a(2*n-1), a(2*n+1) = 3*a(2*n) - 1, with a(1) = 1.

Terms

a(0) =1a(1) =3a(2) =8a(3) =24a(4) =71a(5) =213a(6) =638a(7) =1914a(8) =5741a(9) =17223a(10) =51668a(11) =155004a(12) =465011a(13) =1395033a(14) =4185098a(15) =12555294a(16) =37665881a(17) =112997643a(18) =338992928a(19) =1016978784a(20) =3050936351a(21) =9152809053a(22) =27458427158a(23) =82375281474a(24) =247125844421a(25) =741377533263