Number of tree-like heptagonal systems.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =26a(5) =126a(6) =640a(7) =3559a(8) =20366a(9) =120535a(10) =727292a(11) =4468614a(12) =27842644a(13) =175644673a(14) =1119847820a(15) =7207062667a(16) =46769169830a(17) =305762838483