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partition matroid : ウィキペディア英語版 | partition matroid In mathematics, a partition matroid or partitional matroid is a matroid formed from a direct sum of uniform matroids.〔.〕 ==Definition== Let be a collection of disjoint sets, and let be integers with . Define a set to be "independent" when, for every index , . Then the sets that are independent sets in this way form the independent sets of a matroid, called a partition matroid. The sets are called the blocks of the partition matroid. A basis of the matroid is a set whose intersection with every block has size exactly , and a circuit of the matroid is a subset of a single block with size exactly . The rank of the matroid is .〔.〕 Every uniform matroid is a partition matroid, with a single block of elements and with . Every partition matroid is the direct sum of a collection of uniform matroids, one for each of its blocks. In some publications, the notion of a partition matroid is defined more restrictively, with every . The partitions that obey this more restrictive definition are the transversal matroids of the family of disjoint sets given by their blocks.〔E.g., see . uses the broader definition but notes that the restriction is useful in many applications.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「partition matroid」の詳細全文を読む
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