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

Terms

a(0) =0a(1) =0a(2) =1a(3) =0a(4) =1a(5) =0a(6) =0a(7) =-2a(8) =-5a(9) =-12a(10) =-25a(11) =-50a(12) =-96a(13) =-180a(14) =-331a(15) =-600a(16) =-1075a(17) =-1908a(18) =-3360a(19) =-5878a(20) =-10225a(21) =-17700a(22) =-30509a(23) =-52390a(24) =-89664a(25) =-153000a(26) =-260375a(27) =-442032a(28) =-748775a(29) =-1265832