Bisection of A013697.

Terms

a(0) =4a(1) =27a(2) =119a(3) =497a(4) =2023a(5) =8149a(6) =32692a(7) =130938a(8) =524050a(9) =2096730a(10) =8387859a(11) =33553102a(12) =134215364a(13) =536866711a(14) =2147476180a(15) =8589921317a(16) =34359714770a(17) =137438911520a(18) =549755739309