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Paper   IPM / M / 8742
School of Mathematics
  Title:   Inclusion matrices and chains
1.  E. Ghorbani
2.  G. B. Khosrovshahi
3.  Ch. Maysoori
4.  M. Mohammad-Noori
  Status:   Published
  Journal: J. Combin. Theory Ser. A
  Vol.:  115
  Year:  2008
  Pages:   878-887
  Supported by:  IPM
Given integers t, k, and v such that 0 ≤ tkv, let Wtk(v) be the inclusion matrix of t-subsets vs. k-subsets of a v-set. We modify slightly the concept of standard tableau to study the notion of rank of a finite set of positive integers which was introduced by Frankl. Utilizing this, a decomposition of the poset 2[v] into symmetric skipless chains is given. Based on this decomposition, we construct an inclusion matrix, denoted by Wtk(v), which is row-equivalent to Wtk(v). Its Smith normal form is determined. As applications, Wilson's diagonal form of Wtk(v) is obtained as well as a new proof of the well known theorem on the necessary and sufficient conditions for existence of integral solutions of the system Wtkx=b due to Wilson. Finally we present anotherinclusion matrix with similar properties to those of Wtk(v) which is in some way equivalent to Wtk(v).

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