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

Terms

a(0) =1a(1) =1a(2) =6a(3) =21a(4) =81a(5) =306a(6) =1161a(7) =4401a(8) =16686a(9) =63261a(10) =239841a(11) =909306a(12) =3447441a(13) =13070241a(14) =49553046a(15) =187869861a(16) =712268721a(17) =2700415746a(18) =10238053401a(19) =38815407441a(20) =147160382526a(21) =557927369901