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

Terms

a(0) =0a(1) =0a(2) =1a(3) =0a(4) =2a(5) =6a(6) =15a(7) =46a(8) =132a(9) =380a(10) =1101a(11) =3180a(12) =9190a(13) =26562a(14) =76763a(15) =221850a(16) =641160a(17) =1852984a(18) =5355225a(19) =15476888a(20) =44729034a(21) =129269310a(22) =373595239a(23) =1079710278a(24) =3120420620a(25) =9018182964a(26) =26063032485a(27) =75323561860a(28) =217689133998a(29) =629133273722