a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4).

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =2a(5) =2a(6) =4a(7) =8a(8) =18a(9) =34a(10) =68a(11) =136a(12) =274a(13) =546a(14) =1092a(15) =2184a(16) =4370a(17) =8738a(18) =17476a(19) =34952a(20) =69906a(21) =139810a(22) =279620a(23) =559240a(24) =1118482a(25) =2236962a(26) =4473924a(27) =8947848a(28) =17895698a(29) =35791394