a(n) = 2*a(n-1) + 2*a(n-2) + 2*a(n-3) - a(n-4); a(0)=0, a(1)=1, a(2)=2, a(3)=5.

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =16a(5) =45a(6) =130a(7) =377a(8) =1088a(9) =3145a(10) =9090a(11) =26269a(12) =75920a(13) =219413a(14) =634114a(15) =1832625a(16) =5296384a(17) =15306833a(18) =44237570a(19) =127848949a(20) =369490320a(21) =1067846845a(22) =3086134658a(23) =8919094697a(24) =25776662080a(25) =74495936025a(26) =215297250946a(27) =622220603405