8^n+4^n-1^n.

Terms

a(0) =1a(1) =11a(2) =79a(3) =575a(4) =4351a(5) =33791a(6) =266239a(7) =2113535a(8) =16842751a(9) =134479871a(10) =1074790399a(11) =8594128895a(12) =68736253951a(13) =549822922751