a(0),...,a(3) = 1, 2, 4, 8; thereafter a(n) = a(n-1) + 2*a(n-2) + 4*a(n-3) + 8*a(n-4), n>3.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =32a(5) =80a(6) =208a(7) =560a(8) =1552a(9) =4144a(10) =11152a(11) =30128a(12) =81424a(13) =219440a(14) =592016a(15) =1597616a(16) =4310800a(17) =11629616a(18) =31377808a(19) =84661168a(20) =228421648a(21) =616292144a(22) =1662802576a(23) =4486362800a(24) =12104509712a(25) =32658782768a(26) =88115674000a(27) =237742180784