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A059106
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Number of solutions to variant of Langford (or Langford-Skolem) problem.
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4
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1, 0, 0, 3, 5, 0, 0, 252, 1328, 0, 0, 227968, 1520280, 0, 0, 700078384, 6124491248, 0, 0, 5717789399488, 61782464083584, 0, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,4
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COMMENTS
| How many ways are of arranging the numbers 1,1,2,2,3,3,...,n,n so that there are zero numbers between the two 1's, one number between the two 2's, ..., n-1 numbers between the two n's?
For n>1, a(n)=A004075(n)/2 because A004075 also counts reflected solutions. - Martin Fuller (martin_n_fuller(AT)btinternet.com), Mar 08 2007
Due to symmetry, is a(5) = 5 the largest prime in this sequence? [Jonathan Vos Post, Apr 2, 2011]
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REFERENCES
| R. S. Nickerson, A variant of Langford's Problem, American Math. Monthly, 1967, 74, 591-595.
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LINKS
| J. E. Miller, Langford's Problem
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EXAMPLE
| For n=4 a solution is 42324311.
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CROSSREFS
| Cf. A014552, A050998, A059107, A059108.
Cf. A004075.
Sequence in context: A025115 A113037 A063866 * A087676 A058813 A132701
Adjacent sequences: A059103 A059104 A059105 * A059107 A059108 A059109
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KEYWORD
| nonn,nice,hard
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Feb 14 2001
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EXTENSIONS
| a(20) - a(23) from Mike Godfrey (m.godfrey(AT)umist.ac.uk), Mar 14 2002
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