a(0)=0, a(1)=1, a(2)=2; for n > 2, a(n) = 3*a(n-1) - a(n-2) - 2*a(n-3).

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =11a(5) =24a(6) =51a(7) =107a(8) =222a(9) =457a(10) =935a(11) =1904a(12) =3863a(13) =7815a(14) =15774a(15) =31781a(16) =63939a(17) =128488a(18) =257963a(19) =517523a(20) =1037630a(21) =2079441a(22) =4165647a(23) =8342240a(24) =16702191a(25) =33433039a(26) =66912446a(27) =133899917a(28) =267921227a(29) =536038872