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

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =21a(5) =61a(6) =183a(7) =543a(8) =1629a(9) =4873a(10) =14619a(11) =43815a(12) =131445a(13) =394213a(14) =1182639a(15) =3547551a(16) =10642653a(17) =31926873a(18) =95780619a(19) =287338599a(20) =862015797a(21) =2586037645a(22) =7758112935a(23) =23274309567a(24) =69822928701a(25) =209468698473a(26) =628406095419