Combinatorics (Fall 2010)/Finite set systems

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Systems of Distinct Representatives (SDR)

Hall's marriage theorem

Hall's Theorem
The sets [math]\displaystyle{ S_1,S_2,\ldots,S_m }[/math] have a system of distinct representatives (SDR) if and only if
[math]\displaystyle{ \left|\bigcup_{i\in I}S_i\right|\ge |I| }[/math] for all [math]\displaystyle{ I\subseteq\{1,2,\ldots,m\} }[/math].

2-factor of regular graph

Theorem (Peterson 1891)
Every regular graph with positive even degree has a 2-factor.

Doubly stochastic matrices

Theorem (Birkhoff 1949; von Neumann 1953)
Every doubly stochastic matrix is a convex combination of permutation matrices.

Cuckoo hashing

Min-max theorems

  • König-Egerváry theorem
  • Menger's theorem
  • Dilworth's theorem


Theorem (König 1931; Egerváry 1931)
In any bipartite graph, the size of a maximum matching equals the size of a minimum vertex cover.
Theorem (Menger 1927)
Let [math]\displaystyle{ G }[/math] be a graph and let [math]\displaystyle{ s }[/math] and [math]\displaystyle{ t }[/math] be two vertices of [math]\displaystyle{ G }[/math]. The maximum number of internally disjoint paths from [math]\displaystyle{ s }[/math] to [math]\displaystyle{ t }[/math] equals the minimum number of vertices in a[math]\displaystyle{ s }[/math]-[math]\displaystyle{ t }[/math] separating set.

Chains and Anti-chains

Dilworth's theorem

Sperner's Theorem