Number of P-graphs with vertical symmetry.

Terms

a(0) =1a(1) =3a(2) =10a(3) =39a(4) =158a(5) =674a(6) =2944a(7) =13191a(8) =60078a(9) =277890a(10) =1300536a(11) =6151078a(12) =29346036a(13) =141088632a(14) =682825140a(15) =3324172359a(16) =16267134462a(17) =79975707362a(18) =394833880540