a(n) = a(n-1) + a(n-2) + 2a(n-3).

Terms

a(0) =1a(1) =-2a(2) =4a(3) =4a(4) =4a(5) =16a(6) =28a(7) =52a(8) =112a(9) =220a(10) =436a(11) =880a(12) =1756a(13) =3508a(14) =7024a(15) =14044a(16) =28084a(17) =56176a(18) =112348a(19) =224692a(20) =449392a(21) =898780a(22) =1797556a(23) =3595120a(24) =7190236a(25) =14380468a(26) =28760944a(27) =57521884a(28) =115043764a(29) =230087536