791 research outputs found
Bipartite induced density in triangle-free graphs
We prove that any triangle-free graph on vertices with minimum degree at
least contains a bipartite induced subgraph of minimum degree at least
. This is sharp up to a logarithmic factor in . Relatedly, we show
that the fractional chromatic number of any such triangle-free graph is at most
the minimum of and as . This is
sharp up to constant factors. Similarly, we show that the list chromatic number
of any such triangle-free graph is at most as
.
Relatedly, we also make two conjectures. First, any triangle-free graph on
vertices has fractional chromatic number at most
as . Second, any triangle-free
graph on vertices has list chromatic number at most as
.Comment: 20 pages; in v2 added note of concurrent work and one reference; in
v3 added more notes of ensuing work and a result towards one of the
conjectures (for list colouring
Semialgebraic Graphs having Countable List-Chromatic Numbers
The set of semialgebraic graphs having countable list-chromatic numbers is
characterized. Some other related sets of graphs having countable
list-chromatic numbers also are.Comment: This version has been completely rewritten. It will appear in PAM
Chromatic number of Euclidean plane
If the chromatic number of Euclidean plane is larger than four, but it is
known that the chromatic number of planar graphs is equal to four, then how
does one explain it? In my opinion, they are contradictory to each other. This
idea leads to confirm the chromatic number of the plane about its exact value
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