Bisection of A001317.

Terms

a(0) =3a(1) =15a(2) =51a(3) =255a(4) =771a(5) =3855a(6) =13107a(7) =65535a(8) =196611a(9) =983055a(10) =3342387a(11) =16711935a(12) =50529027a(13) =252645135a(14) =858993459a(15) =4294967295a(16) =12884901891a(17) =64424509455a(18) =219043332147