Related to series-parallel networks.

Terms

a(0) =1a(1) =5a(2) =13a(3) =45a(4) =121a(5) =413a(6) =1261a(7) =4221a(8) =13801a(9) =46365a(10) =155497a(11) =527613a(12) =1792805a(13) =6126293a(14) =20986153a(15) =72121853a(16) =248396793a(17) =857416949a(18) =2964896877a(19) =10269596445a(20) =35622421561a(21) =123728022269a(22) =430254861945