Number of symmetric irreducible diagrams with 2n nodes.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =22a(5) =96a(6) =380a(7) =1853a(8) =8510a(9) =44940a(10) =229836a(11) =1296410a(12) =7211116a(13) =43096912a(14) =256874200a(15) =1617413773a(16) =10226972110a(17) =67542201972a(18) =449809389740