In combinatorial mathematics, a Langford pairing, also called a Langford sequence, is a permutation of the sequence of 2n numbers 1, 1, 2, 2, ..., n, n in which the two ones are one unit apart, the two twos are two units apart, and more generally the two copies of each number k are k units apart. Langford pairings are named after C. Dudley Langford, who posed the problem of constructing them in 1958.
Langford's problem is the task of finding Langford pairings for a given value of n.
The closely related concept of a Skolem sequence is defined in the same way, but instead permutes the sequence 0, 0, 1, 1, ..., n − 1, n − 1.
For example, a Langford pairing for n = 3 is given by the sequence 2,3,1,2,1,3.
Langford pairings exist only when n is congruent to 0 or 3 modulo 4; for instance, there is no Langford pairing when n = 1, 2, or 5.
The numbers of different Langford pairings for n = 1, 2, …, counting any sequence as being the same as its reversal, are
As Knuth (2008) describes, the problem of listing all Langford pairings for a given n can be solved as an instance of the exact cover problem, but for large n the number of solutions can be calculated more efficiently by algebraic methods.
- Stirling permutation, a different type of permutation of the same multiset
- Gardner, Martin (1978), "Langford's problem", Mathematical Magic Show, Vintage, p. 70.
- Knuth, Donald E. (2008), The Art of Computer Programming, Vol. IV, Fascicle 0: Introduction to Combinatorial Algorithms and Boolean Functions, Addison-Wesley, ISBN 978-0-321-53496-5.
- Langford, C. Dudley (1958), "Problem", Mathematical Gazette, 42: 228.
- Nordh, Gustav (2008), "Perfect Skolem sets", Discrete Mathematics, 308 (9): 1653–1664, arXiv: , doi:10.1016/j.disc.2006.12.003, MR 2392605.
- Skolem, Thoralf (1957), "On certain distributions of integers in pairs with given differences", Mathematica Scandinavica, 5: 57–68, MR 0092797.