a(n) = a(n-1) + a(n-2) + a(n-3) + 2*a(n-4) with a(0)=2, a(1)=1, a(2)=5, a(3)=10.

Terms

a(0) =2a(1) =1a(2) =5a(3) =10a(4) =20a(5) =37a(6) =77a(7) =154a(8) =308a(9) =613a(10) =1229a(11) =2458a(12) =4916a(13) =9829a(14) =19661a(15) =39322a(16) =78644a(17) =157285a(18) =314573a(19) =629146a(20) =1258292a(21) =2516581a(22) =5033165a(23) =10066330a(24) =20132660a(25) =40265317a(26) =80530637a(27) =161061274a(28) =322122548a(29) =644245093