a(n)=3*a(n-1)-a(n-2) with a(0)=1, a(1)=3, a(2)=11.

Terms

a(0) =1a(1) =3a(2) =11a(3) =30a(4) =79a(5) =207a(6) =542a(7) =1419a(8) =3715a(9) =9726a(10) =25463a(11) =66663a(12) =174526a(13) =456915a(14) =1196219a(15) =3131742a(16) =8199007a(17) =21465279a(18) =56196830a(19) =147125211a(20) =385178803a(21) =1008411198a(22) =2640054791a(23) =6911753175a(24) =18095204734a(25) =47373861027a(26) =124026378347a(27) =324705274014a(28) =850089443695