Trisection of A117981.

Terms

a(0) =1a(1) =7a(2) =73a(3) =511a(4) =3577a(5) =37303a(6) =262657a(7) =1838599a(8) =19173961a(9) =134217727a(10) =939524089a(11) =9797894071a(12) =68585258497a(13) =480096809479