Number of permutations of [1..n] that can be drawn on a circle.

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =84a(5) =308a(6) =1090a(7) =3782a(8) =13000a(9) =44504a(10) =152102a(11) =519506a(12) =1773948a(13) =6056932a(14) =20680010a(15) =70606382a(16) =241065744a(17) =823050480a(18) =2810070734a(19) =9594182314a(20) =32756588164a(21) =111837988444a(22) =381838777906