The OEIS is supported by the many generous donors to the OEIS Foundation.


(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A304271 Number of unrestricted planar Langford sequences. 0


%S 0,0,1,1,0,0,6,24,0,0,139,289,0,0,2414,4455,0,0,33222,63700,0,0,

%T 437489,794953,0,0

%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.

%K nonn,more

%O 1,7

%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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 25 22:51 EDT 2022. Contains 354073 sequences. (Running on oeis4.)