a(n) = 2*(a(n-1)+a(n-2)+a(n-3))-a(n-4) for n >= 4, with initial terms -1, 2, 3, 6.

Terms

a(0) =-1a(1) =2a(2) =3a(3) =6a(4) =23a(5) =62a(6) =179a(7) =522a(8) =1503a(9) =4346a(10) =12563a(11) =36302a(12) =104919a(13) =303222a(14) =876323a(15) =2532626a(16) =7319423a(17) =21153522a(18) =61134819a(19) =176682902a(20) =510623063a(21) =1475728046a(22) =4264933203a(23) =12325885722a(24) =35622470879a(25) =102950851562a(26) =297533483123a(27) =859887725406