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

Terms

a(0) =1a(1) =2a(2) =9a(3) =33a(4) =126a(5) =477a(6) =1809a(7) =6858a(8) =26001a(9) =98577a(10) =373734a(11) =1416933a(12) =5372001a(13) =20366802a(14) =77216409a(15) =292749633a(16) =1109898126a(17) =4207943277a(18) =15953524209a(19) =60484402458a(20) =229313780001a(21) =869394547377