Number of skeins with vertical symmetry.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =24a(5) =96a(6) =388a(7) =1667a(8) =7278a(9) =32726a(10) =149232a(11) =692014a(12) =3244182a(13) =15374906a(14) =73474008a(15) =353835147a(16) =1714967402a(17) =8360511370a(18) =40964207460a(19) =201630767026a(20) =996494168808