a(n) = 4*a(n-1) - 3*a(n-2) - a(n-3), with a(0)=0, a(1)=0 and a(2)=1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =13a(5) =39a(6) =113a(7) =322a(8) =910a(9) =2561a(10) =7192a(11) =20175a(12) =56563a(13) =158535a(14) =444276a(15) =1244936a(16) =3488381a(17) =9774440a(18) =27387681a(19) =76739023a(20) =215018609a(21) =602469686a(22) =1688083894a(23) =4729907909a(24) =13252910268a(25) =37133833451a(26) =104046695091a(27) =291532369743a(28) =816855560248