# Selection theorems

A group of theorems in combinatorial theory related to the selection of elements from a set which in some way correspond to a family of subsets of that set. Selection theorems are commonly employed as existence theorems in solving various combinatorial problems. Below some of the most important selection theorems are listed, and examples of their application are given.

1) Let be some family of subsets of a given set . A sample of distinct elements of the set is called a system of distinct representatives (cf. System of different representatives) of the family if , ; the element is a representative of the set . For example, if and consists of , , , , then is a system of distinct representatives of , where the element 5 represents the set , the element 2 represents set , etc. If, on the other hand, is composed of sets , , , , will have no system of distinct representatives, since together contain only three elements.

The theorem on a system of distinct representatives: A family has a system of distinct representatives if and only if the union of any sets of contains at least distinct elements, where .

This theorem was proved by Ph. Hall [3] (see also [1], [2]). It may be used to prove the theorem on common representatives, which is also a selection theorem. Let

(1) |

(2) |

be two partitions of the set , i.e. none of the components is empty and if . The set is called a system of common representatives of the partitions (1) and (2) if is a system of distinct representatives both for the family and the family . For instance, if and if

are two partitions of , where , , , , , , , , then is a system of common representatives of the families and since it is a system of distinct representatives for both and ; here the element 0 represents the sets and , the element 6 represents and , the element 7 represents and , while the element 9 represents and .

The theorem on a system of common representatives: The partitions (1) and (2) have a system of common representatives if and only if the union of any sets of contains at most sets from the family , [1], [2].

2) Let there be given a rectangular matrix. The term line will denote both a row and a column of this matrix.

König's theorem: If the elements of a rectangular matrix are zeros and ones, the minimum number of lines containing all ones is equal to the maximum number of ones which may be selected so that no two of them are located on the same line.

This theorem was formulated and proved by D. König ([4], see also [1], [2]). It is equivalent to the Ph. Hall theorem. It is employed, for example, in order to prove that certain matrices are linear combinations of permutation matrices (a permutation matrix is a rectangular matrix of dimension , consisting of zeros and ones, such that , where is the transposed matrix of and is the identity matrix of order ; for example, a square permutation matrix of order consists of ones which are so disposed that no two of them lie on the same line). In other words, if a matrix of dimension , , has been given, with non-negative real numbers as its elements and such that the sum of the elements of each row in is and the sum of the elements of each column is , then

where each is a permutation matrix, while the coefficients are non-negative real numbers [1], [2]. In particular, if a square matrix of order , consisting of zeros and ones, is such that the sum of the elements in any column or any row is equal to a positive integer , then

where all are permutation matrices of order .

Let be a finite set and let be the set of all its subsets containing exactly elements. Let

(3) |

be an arbitrary ordered partition of into components . Let be integers such that

(4) |

If there exists a subset, containing elements of the set , such that all its subsets containing exactly elements are contained in , it is said to be a -subset of the set .

Ramsey's theorem: Let there be given integers and which satisfy condition (4). Then there exists a natural number such that for any integer the following assertion is valid: Given a set of elements and an arbitrary ordered partition (3) of the set into components , then will contain a -subset for some .

This theorem was proved by F. Ramsey [5]; see also [1], [2]. An example of an application of this theorem is the following result [6], [1], [2]: For any given integer there exists an integer such that out of points in a plane no three of which lie on a straight line, there are points which form a convex -gon.

#### References

[1] | M. Hall, "Combinatorial theory" , Blaisdell (1967) |

[2] | H.J. Ryser, "Combinatorial mathematics" , Carus Math. Monogr. , 14 , Math. Assoc. Amer. (1963) |

[3] | Ph. Hall, "On representatives of subsets" J. London Math. Soc. , 1 (1935) pp. 26–30 |

[4] | D. König, "Theorie der endlichen und unendlichen Graphen" , Chelsea, reprint (1950) |

[5] | F.P. Ramsey, "On a problem of formal logic" Proc. London Math. Soc. (2) , 30 (1930) pp. 264–286 |

[6] | P. Erdös, G. Szekeres, "A combinatorial problem in geometry" Compositio Math. , 2 (1935) pp. 463–470 |

#### Comments

The popular name of the Ph. Hall theorem is marriage theorem or Ph. Hall marriage theorem.

An ordered partition of (as in (3)) can be regarded as a colouring of (with colours).

An important selection theorem is Rado's selection principle, see [a1]–[a3].

Selection problems and theorems arise in many parts of mathematics, not only combinatorics. The general setting is that of a set-valued mapping (where is the set of all subsets of ) and the problem is to find a selection such that for all . Such a function is sometimes called a selector.

Note that the axiom of choice is an assertion about the existence of selections.

Further selection problems occur in topology, game theory, probability, measure theory, analysis, etc. A selection follows.

The Kuratowski–Ryll-Nardzweski selection theorem, [a1], says the following. Let be a space with a -algebra of subsets (cf. Algebra of sets) and let be a complete separable metric space. Let be a measurable set-valued function on to the closed non-empty subsets of . Here, measurable means that is in for every open . Then there exists a measurable selector (i.e. such that for every open ).

The von Neumann measurable choice theorem, [a2], says essentially the following. Let be a complete separable metric space and an analytic set-valued function. Then there is a Lebesgue-measurable selector . Here, the set-valued function on is called analytic if its graph is an analytic subset of (cf. Analytic set).

In topology there is, for example, Michael's continuous selection theorem, [a3], which characterizes paracompactness: A -space is paracompact if and only if every lower semi-continuous mapping on with values in the closed convex non-empty subsets of a Banach space admits a continuous selector.

Cf. [a4], [a5] for some more selection theorems, more details and applications. For some other (variants of) selection theorems cf. also Multi-valued mapping. The phrase "selection theorem" is also used for various results pertaining e.g. to the selection of converging subsequences, cf. e.g. Helly theorem; Blaschke selection theorem; Bolzano–Weierstrass selection principle.

#### References

[a1] | K. Kuratowski, C. Ryll-Nardzweski, "A general theorem on selectors" Bull. Acad. Pol. Sci., Ser. Math. Astron. Phys. , 13 (1965) pp. 397–403 |

[a2] | J. von Neumann, "On rings of operators. Reduction theory" Ann. of Math. , 50 (1949) pp. 448–451 |

[a3] | E. Michael, "Continuous selections I" Ann. of Math. , 63 (1956) pp. 361–382 |

[a4] | T. Parthasarathy, "Selection theorems and their applications" , Lect. notes in math. , 263 , Springer (1972) |

[a5] | W.M. Fleischman (ed.) , Set valued mappings, selections and topological properties of (Proc. Conf. SUNY Buffalo, 1969) , Lect. notes in math. , 171 , Springer (1970) |

[a6] | L. Mirsky, "Transversal theory" , Acad. Press (1971) |

[a7] | H. Lüneburg, "Tools and fundamental constructions of combinatorial mathematics" , B.I. Wissenschaftsverlag Mannheim (1989) |

[a8] | R.L. Graham, B.L. Rothschild, J.H. Spencer, "Ramsey theory" , Wiley (Interscience) (1980) |

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Selection theorems.

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