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On Perfect Balanced Rainbow-Free Colorings and Complete Colorings of Projective Spaces

  • Lijun Ma,
  • Zihong Tian

摘要

This paper is motivated by the problem of determining the related chromatic numbers of some hypergraphs. A hypergraph \(\Pi _{q}(n,k)\) Π q ( n , k ) is defined from a projective space PG \((n-1,q)\) ( n - 1 , q ) , where the vertices are points and the hyperedges are \((k-1)\) ( k - 1 ) -dimensional subspaces. For the perfect balanced rainbow-free colorings, we show that \({\overline{\chi }}_{p}(\Pi _{q}(n,k))=\frac{q^n-1}{l(q-1)}\) χ ¯ p ( Π q ( n , k ) ) = q n - 1 l ( q - 1 ) , where \(k\ge \lceil \frac{n+1}{2}\rceil \) k n + 1 2 and l is the smallest nontrivial factor of \(\frac{q^n-1}{q-1}\) q n - 1 q - 1 . For the complete colorings, we prove that there is no complete coloring for \(\Pi _{q}(n,k)\) Π q ( n , k ) with \(2\le k<n\) 2 k < n . We also provide some results on the related chromatic numbers of subhypergraphs of \(\Pi _{q}(n,k)\) Π q ( n , k ) .