a(n) = 2*(a(n-1)+a(n-2)+a(n-3))-a(n-4) for n >= 4, with initial terms 0, 1, 1, 0.

Terms

a(0) =0a(1) =1a(2) =1a(3) =0a(4) =4a(5) =9a(6) =25a(7) =76a(8) =216a(9) =625a(10) =1809a(11) =5224a(12) =15100a(13) =43641a(14) =126121a(15) =364500a(16) =1053424a(17) =3044449a(18) =8798625a(19) =25428496a(20) =73489716a(21) =212389225a(22) =613816249a(23) =1773961884a(24) =5126845000a(25) =14816857041a(26) =42821511601a(27) =123756465400a(28) =357662823084