Arises in study of A000587.

Terms

a(0) =1a(1) =1a(2) =1a(3) =5a(4) =13a(5) =13a(6) =13a(7) =77a(8) =77a(9) =333a(10) =845a(11) =1869a(12) =3917a(13) =8013a(14) =8013a(15) =24397a(16) =57165a(17) =122701a(18) =122701