a(n) = 6*a(n-1) - a(n-2), n >= 2, a(0)=1, a(1)=4.

Terms

a(0) =1a(1) =4a(2) =23a(3) =134a(4) =781a(5) =4552a(6) =26531a(7) =154634a(8) =901273a(9) =5253004a(10) =30616751a(11) =178447502a(12) =1040068261a(13) =6061962064a(14) =35331704123a(15) =205928262674