Graph Theory » Coloring » Vertex coloring

Oriented chromatic number of planar graphs ★★

Author(s):

An oriented colouring of an oriented graph is assignment $ c $ of colours to the vertices such that no two arcs receive ordered pairs of colours $ (c_1,c_2) $ and $ (c_2,c_1) $. It is equivalent to a homomorphism of the digraph onto some tournament of order $ k $.

Problem   What is the maximal possible oriented chromatic number of an oriented planar graph?

Keywords: oriented coloring; oriented graph; planar graph

Posted by Robert Samal
updated August 4th, 2007
add new comment
Number Theory » Combinatorial N.T.

Covering systems with big moduli ★★

Author(s): Erdos; Selfridge

Problem   Does for every integer $ N $ exist a covering system with all moduli distinct and at least equal to~$ N $?

Keywords: covering system

Posted by Robert Samal
updated August 4th, 2007
add new comment
Number Theory » Combinatorial N.T.

Odd incongruent covering systems ★★★

Author(s): Erdos; Selfridge

Conjecture   There is no covering system whose moduli are odd, distinct, and greater than 1.

Keywords: covering system

Posted by Robert Samal
updated August 4th, 2007
add new comment
Graph Theory » Infinite Graphs

Hamiltonian cycles in powers of infinite graphs ★★

Author(s): Georgakopoulos

Conjecture  
    \item If $ G $ is a countable connected graph then its third power is hamiltonian. \item If $ G $ is a 2-connected countable graph then its square is hamiltonian.

Keywords: hamiltonian; infinite graph

Posted by Robert Samal
updated July 24th, 2007
add new comment