%N Number of unrestricted planar Langford sequences.
%C Enumerates the Langford sequences (counted by A014552) that are planar in a sense more general than the one used by A125762. In that sequence the noncrossing joining lines are each restricted to lie in one of the two half-planes separated by the axis of the numerical sequence. Here we allow the joining lines to use the whole plane, requiring them only to be noncrossing and not to pass between the terms of the Langford sequence.
%D D. E. Knuth, TAOCP, Vol. 4, in preparation.
%H John E. Miller, <a href="http://dialectrix.com/langford.html">Langford's Problem</a>
%e When n=4 the Langford sequence 23421314 is not planar in the sense of A125762, but is planar in the sense of this sequence: the line that joins the 3s does not lie entirely "above" or "below" the numerical array but passes around the end of the array.
%Y Cf. A014552, A125762.
%A _Rory Molinari_, May 09 2018
%E a(23) from _Rory Molinari_, Jun 04 2019
%E a(24)-a(26) from _Rory Molinari_, Dec 02 2019