Arrange copies of the  digits 1, ...,  such that there is one digit between the 1s, two digits between the 2s, etc. For example, the unique (modulo reversal)  solution is 231213, and the unique (again modulo reversal)  solution is 23421314. Solutions to Langford's problem exist only if , so the next solutions occur for . There are 26 of these, as exhibited by Lloyd (1971). In lexicographically smallest order (i.e., small digits come first), the first few Langford sequences are 231213, 23421314, 14156742352637, 14167345236275, 15146735423627, ... (OEIS A050998).

The number of solutions for , 4, 5, ... (modulo reversal of the digits) are 1, 1, 0, 0, 26, 150, 0, 0, 17792, 108144, ... (OEIS A014552). No formula is known for the number of solutions of a given order .

REFERENCES:

Davies, R. O. "On Langford's Problem. II." Math. Gaz. 43, 253-255, 1959.

Gardner, M. Mathematical Magic Show: More Puzzles, Games, Diversions, Illusions and Other Mathematical Sleight-of-Mind from Scientific American. New York: Vintage, pp. 70 and 77-78, 1978.

Langford, C. D. "Problem." Math. Gaz. 42, 228, 1958.

Lloyd, P. R. Correspondence to the Editor. Math. Gaz. 55, 73, 1971.

Lorimer, P. "A Method of Constructing Skolem and Langford Sequences." Southeast Asian Bull. Math. 6, 115-119, 1982.

Miller, J. "Langford's Problem." https://www.lclark.edu/~miller/langford.html.

Miller, J. "Langford's Problem Bibliography." https://www.lclark.edu/~miller/langford/langford-biblio.html.

Simpson, J. E. "Langford Sequences: Perfect and Hooked." Disc> Math. 44, 97-104, 1983.

Priday, C. J. "On Langford's Problem. I." Math. Gaz. 43, 250-253, 1959.

Sloane, N. J. A. Sequences A014552 and A050998 in "The On-Line Encyclopedia of Integer Sequences."