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

Terms

a(0) =2a(1) =6a(2) =19a(3) =61a(4) =197a(5) =633a(6) =2036a(7) =6548a(8) =21061a(9) =67737a(10) =217861a(11) =700699a(12) =2253638a(13) =7248306a(14) =23312507a(15) =74979305a(16) =241153657a(17) =775615161a(18) =2494587424a(19) =8023265560a(20) =25804984685a(21) =82995786381a(22) =266936820209a(23) =858540765611