a(n) = 3*a(n-1) + a(n-2), with a(1)=1 and a(2)=4.

Terms

a(0) =1a(1) =4a(2) =13a(3) =43a(4) =142a(5) =469a(6) =1549a(7) =5116a(8) =16897a(9) =55807a(10) =184318a(11) =608761a(12) =2010601a(13) =6640564a(14) =21932293a(15) =72437443a(16) =239244622a(17) =790171309a(18) =2609758549a(19) =8619446956a(20) =28468099417a(21) =94023745207a(22) =310539335038