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

Terms

a(0) =1a(1) =8a(2) =33a(3) =140a(4) =593a(5) =2512a(6) =10641a(7) =45076a(8) =190945a(9) =808856a(10) =3426369a(11) =14514332a(12) =61483697a(13) =260449120a(14) =1103280177a(15) =4673569828a(16) =19797559489a(17) =83863807784a(18) =355252790625