Langford's Problem
Arrange copies of the 
digits 1, ..., such that there is one digit between the 1s, two digits between the 2s, etc. For example, the 
solution is 312132 and the 
solution is 41312432. Solutions exist only if 
. Thenumber of solutions for , 4, 5, ... are 1, 1, 0, 0, 26, 150, 0, 0, 17792, 108144, ... (Sloane's A014552).
References
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. Sloane, N. J. A. Sequence A014552in ``The On-Line Version of the Encyclopedia of Integer Sequences.''http://www.research.att.com/~njas/sequences/eisonline.html.
- Christoffel Symbol of the Second Kind
- Chromatic Number
- Chromatic Polynomial
- Chu Identity
- Church's Theorem
- Church's Thesis
- Church-Turing Thesis
- Chu Space
- Chu-Vandermonde Identity
- Chvátal's Art Gallery Theorem
- Chvátal's Theorem
- ci
- Ci
- Cigarettes
- Cin
- Circle
- Circle Caustic
- Circle-Circle Intersection
- Circle Cutting
- Circle Evolute
- Circle Inscribing
- Circle Involute
- Circle Involute Pedal Curve
- Circle Lattice Points
- Circle Lattice Theorem