a(n) = 2*a(n-1) + a(n-2); a(0)=1, a(1)=4.

Terms

a(0) =1a(1) =4a(2) =9a(3) =22a(4) =53a(5) =128a(6) =309a(7) =746a(8) =1801a(9) =4348a(10) =10497a(11) =25342a(12) =61181a(13) =147704a(14) =356589a(15) =860882a(16) =2078353a(17) =5017588a(18) =12113529a(19) =29244646a(20) =70602821a(21) =170450288a(22) =411503397a(23) =993457082a(24) =2398417561a(25) =5790292204