Baguenaudier
A Puzzle involving disentangling a set of rings from a looped double rod (also called Chinese Rings). Theminimum number of moves needed for 
rings is
By simultaneously moving the two end rings, the number of moves can be reduced to
The solution of the baguenaudier is intimately related to the theory of Gray Codes.
References
Dubrovsky, V. ``Nesting Puzzles, Part II: Chinese Rings Produce a Chinese Monster.'' Quantum 6, 61-65 (Mar.) and 58-59 (Apr.), 1996. Gardner, M. ``The Binary Gray Code.'' In Knotted Doughnuts and Other Mathematical Entertainments. New York: W. H. Freeman, pp. 15-17, 1986. Kraitchik, M. ``Chinese Rings.'' §3.12.3 in Mathematical Recreations. New York: W. W. Norton, pp. 89-91, 1942. Steinhaus, H. Mathematical Snapshots, 3rd American ed. New York: Oxford University Press, p. 268, 1983.
- Heilbronn Triangle Problem
- Heine-Borel Theorem
- Heine Hypergeometric Series
- Heisenberg Group
- Heisenberg Space
- Held Group
- Helen of Geometers
- Helicoid
- Helix
- Helly Number
- Helly's Theorem
- Helmholtz Differential Equation
- Helmholtz Differential Equation--Bipolar Coordinates
- Helmholtz Differential Equation--Cartesian Coordinates
- Helmholtz Differential Equation--Circular Cylindrical Coordinates
- Helmholtz Differential Equation--Confocal Ellipsoidal Coordinates
- Helmholtz Differential Equation--Confocal Paraboloidal Coordinates
- Helmholtz Differential Equation--Conical Coordinates
- Helmholtz Differential Equation--Elliptic Cylindrical Coordinates
- Helmholtz Differential Equation--Oblate Spheroidal Coordinates
- Helmholtz Differential Equation--Parabolic Coordinates
- Helmholtz Differential Equation--Parabolic Cylindrical Coordinates
- Helmholtz Differential Equation Polar Coordinates
- Helmholtz Differential Equation--Prolate Spheroidal Coordinates
- Helmholtz Differential Equation--Spherical Coordinates