

A014552


Number of solutions to Langford (or LangfordSkolem) problem (up to reversal of the order).


11



0, 0, 1, 1, 0, 0, 26, 150, 0, 0, 17792, 108144, 0, 0, 39809640, 326721800, 0, 0, 256814891280, 2636337861200, 0, 0, 3799455942515488, 46845158056515936, 0, 0, 111683606778027803456
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OFFSET

1,7


COMMENTS

These are also called Langford pairings.
2*a(n)=A176127(n) gives the number of ways are of arranging the numbers 1,1,2,2,3,3,...,n,n so that there is one number between the two 1's, two numbers between the two 2's, ..., n numbers between the two n's.
a(n) > 0 iff n == 0 or 3 mod 4.


REFERENCES

Jaromir Abrham, "Exponential lower bounds for the numbers of Skolem and extremal Langford sequences," Ars Combinatoria 22 (1986), 187198.
M. Gardner, Mathematical Magic Show, New York: Vintage, pp. 70 and 7778, 1978.
M. Gardner, Mathematical Magic Show, Revised edition published by Math. Assoc. Amer. in 1989. Contains a postscript on pp. 283284 devoted to a discussion of early computations of the number of Langford sequences.
R. K. Guy, The unity of combinatorics, Proc. 25th Iranian Math. Conf, Tehran, (1994), Math. Appl 329 129159, Kluwer Dordrecht 1995, Math. Rev. 96k:05001.
D. E. Knuth, The Art of Computer Programming, Vol. 4A, Section 7.1.1, p. 2.
M. Krajecki, Christophe Jaillet and Alain Bui, "Parallel tree search for combinatorial problems: A comparative study between OpenMP and MPI," Studia Informatica Universalis 4 (2005), 151190.
Roselle, David P. Distributions of integers into stuples with given differences. Proceedings of the Manitoba Conference on Numerical Mathematics (Univ. Manitoba, Winnipeg, Man., 1971), pp. 3142.Dept. Comput. Sci., Univ. Manitoba, Winnipeg, Man., 1971. MR0335429 (49 #211).  From N. J. A. Sloane, Jun 05 2012


LINKS

Table of n, a(n) for n=1..27.
R. O. Davies, On Langford's problem II, Math. Gaz., 1959, vol. 43, 253255.
M. Krajecki, L(2,23)=3,799,455,942,515,488.
C. D. Langford, 2781. Parallelograms with Integral Sides and Diagonals, Math. Gaz., 1958, vol. 42, p. 228.
J. E. Miller, Langford's Problem
G. Nordh, Perfect Skolem sequences
C. J. Priday, On Langford's Problem I, Math. Gaz., 1959, vol. 43, 250255.
W. Schneider, Langford's Problem
T. Skolem, On certain distributions of integers in pairs with given differences, Math. Scand., 1957, vol. 5, 5768.
T. Saito and S. Hayasaka, Langford sequences: a progress report, Math. Gaz., 1979, vol. 63, #426, 261262.
J. E. Simpson, Langford Sequences: perfect and hooked, Discrete Math., 1983, vol. 44, #1, 97104.
Eric Weisstein's World of Mathematics, Langford's Problem.


FORMULA

a(n) = A176127(n)/2.


EXAMPLE

Solutions for n=3 and 4: 312132 and 41312432. Solution for n=16: 16, 14, 12, 10, 13, 5, 6, 4, 15, 11, 9, 5, 4, 6, 10, 12, 14, 16, 13, 8, 9, 11, 7, 1, 15, 1, 2, 3, 8, 2, 7, 3.


CROSSREFS

See A050998 for further examples of solutions.
If the zeros are omitted we get A192289.
Cf. A059106, A059107, A059108, A125762, A026272.
Sequence in context: A238917 A200449 A248465 * A192289 A230906 A232585
Adjacent sequences: A014549 A014550 A014551 * A014553 A014554 A014555


KEYWORD

nonn,hard,nice,more


AUTHOR

John E. Miller (miller(AT)lclark.edu), Eric W. Weisstein, N. J. A. Sloane


EXTENSIONS

a(20) from Ron van Bruchem and Mike Godfrey, Feb 18 2002
a(21)a(23) sent by John E. Miller (miller(AT)lclark.edu) and Pab Ter (pabrlos(AT)yahoo.com), May 26 2004. These values were found by a team at Universite de Reims ChampagneArdenne, headed by Michael Krajecki, using over 50 processors for 4 days.
a(24)=46845158056515936 was computed circa Apr 15 2005 by the Krajecki team.  Don Knuth, Feb 03 2007
Edited by Max Alekseyev, May 31 2011
a(27) from the J. E. Miller web page "Langford's problem"; thanks to Eric Desbiaux for reporting this.  N. J. A. Sloane, May 18 2015


STATUS

approved



