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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =15a(5) =38a(6) =99a(7) =262a(8) =703a(9) =1904a(10) =5210a(11) =14363a(12) =39858a(13) =111231a(14) =311932a(15) =878504a(16) =2483464a(17) =7044008a(18) =20038990a(19) =57160400a(20) =163443018a(21) =468374024a(22) =1344902554a(23) =3868897760a(24) =11148534334a(25) =32175516814a(26) =92995091280a(27) =269138734764a(28) =779890651873