Dirichlet's Box Principle
A.k.a. the Pigeonhole Principle. Given 
boxes and 
objects, at least one box must contain more than oneobject. This statement has important applications in number theory and was first stated by Dirichlet 
in 1834.
References
Chartrand, G. Introductory Graph Theory. New York: Dover, p. 38, 1985. Shanks, D. Solved and Unsolved Problems in Number Theory, 4th ed. New York: Chelsea, pp. 161, 1993.
- Gregory's Formula
- Grelling's Paradox
- Grenz-Formel
- Griffiths Points
- Griffiths' Theorem
- Grimm's Conjecture
- Grinberg Formula
- Groemer Packing
- Groemer Theorem
- Gronwall's Theorem
- Gross
- Grossman's Constant
- Grothendieck's Constant
- Grothendieck's Theorem
- Ground Set
- Group
- Group Convolution
- Groupoid
- Group Orthogonality Theorem
- Group Ring
- Group Theory
- Growth
- Growth Function
- Growth Spiral
- Grundy's Game