a(n) = 3*a(n-2) - a(n-3), with a(0)=0, a(1)=-3, and a(2)=6.

Terms

a(0) =0a(1) =-3a(2) =6a(3) =-9a(4) =21a(5) =-33a(6) =72a(7) =-120a(8) =249a(9) =-432a(10) =867a(11) =-1545a(12) =3033a(13) =-5502a(14) =10644a(15) =-19539a(16) =37434a(17) =-69261a(18) =131841a(19) =-245217a(20) =464784a(21) =-867492a(22) =1639569a(23) =-3067260a(24) =5786199a(25) =-10841349a(26) =20425857a(27) =-38310246a(28) =72118920a(29) =-135356595