a(n) = -2*a(n-1) + a(n-2) + a(n-3), a(0) = -1, a(1) = 3, a(2) = -9.

Terms

a(0) =-1a(1) =3a(2) =-9a(3) =20a(4) =-46a(5) =103a(6) =-232a(7) =521a(8) =-1171a(9) =2631a(10) =-5912a(11) =13284a(12) =-29849a(13) =67070a(14) =-150705a(15) =338631a(16) =-760897a(17) =1709720a(18) =-3841706a(19) =8632235a(20) =-19396456a(21) =43583441a(22) =-97931103a(23) =220049191a(24) =-494446044a(25) =1111010176a(26) =-2496417205a(27) =5609398542a(28) =-12604204113a(29) =28321389563