G.f. 1/((3*x-1)*(x^2-x-1)).

Terms

a(0) =1a(1) =2a(2) =8a(3) =21a(4) =68a(5) =196a(6) =601a(7) =1782a(8) =5380a(9) =16085a(10) =48344a(11) =144888a(12) =434897a(13) =1304314a(14) =3913552a(15) =11739669a(16) =35220604a(17) =105659228a(18) =316981865a(19) =950938830a(20) =2852827436a(21) =8558464597a(22) =25675422448a(23) =77026220976a(24) =231078737953a(25) =693236092466