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On Uniquely 3-Colorable Plane Graphs without Adjacent Faces of Prescribed Degrees

Author

Listed:
  • Zepeng Li

    (School of Information Science and Engineering, Lanzhou University, Lanzhou 730000, China)

  • Naoki Matsumoto

    (Research Institute for Digital Media and Content, Keio University, Tokyo 108-8345, Japan)

  • Enqiang Zhu

    (Institute of Computing Science and Technology, Guangzhou University, Guangzhou 510006, China)

  • Jin Xu

    (School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China)

  • Tommy Jensen

    (Department of Mathematics, Kyungpook National University, Daegu 41566, Korea)

Abstract

A graph G is uniquely k-colorable if the chromatic number of G is k and G has only one k -coloring up to the permutation of the colors. For a plane graph G , two faces f 1 and f 2 of G are adjacent ( i , j ) -faces if d ( f 1 ) = i , d ( f 2 ) = j , and f 1 and f 2 have a common edge, where d ( f ) is the degree of a face f . In this paper, we prove that every uniquely three-colorable plane graph has adjacent ( 3 , k ) -faces, where k ≤ 5 . The bound of five for k is the best possible. Furthermore, we prove that there exists a class of uniquely three-colorable plane graphs having neither adjacent ( 3 , i ) -faces nor adjacent ( 3 , j ) -faces, where i , j are fixed in { 3 , 4 , 5 } and i ≠ j . One of our constructions implies that there exists an infinite family of edge-critical uniquely three-colorable plane graphs with n vertices and 7 3 n - 14 3 edges, where n ( ≥ 11 ) is odd and n ≡ 2 ( mod 3 ) .

Suggested Citation

  • Zepeng Li & Naoki Matsumoto & Enqiang Zhu & Jin Xu & Tommy Jensen, 2019. "On Uniquely 3-Colorable Plane Graphs without Adjacent Faces of Prescribed Degrees," Mathematics, MDPI, vol. 7(9), pages 1-6, September.
    • Handle: RePEc:gam:jmathe:v:7:y:2019:i:9:p:793-:d:262847
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