Number of elementary arches of size n.

Terms

a(0) =1a(1) =3a(2) =16a(3) =105a(4) =768a(5) =6006a(6) =49152a(7) =415701a(8) =3604480a(9) =31870410a(10) =286261248a(11) =2604681690a(12) =23957864448a(13) =222399744300