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We prove that for any $varepsilon>0$, for any large enough $t$, there is a graph $G$ that admits no $K_t$-minor but admits a $(frac32-varepsilon)t$-colouring that is frozen with respect to Kempe changes, i.e. any two colour classes induce a connected component. This disproves three conjectures of Las Vergnas and Meyniel from 1981.
Hadwiger conjectured in 1943 that for every integer $t ge 1$, every graph with no $K_t$ minor is $(t-1)$-colorable. Kostochka, and independently Thomason, proved every graph with no $K_t$ minor is $O(t(log t)^{1/2})$-colorable. Recently, Postle impro
Hadwigers conjecture is one of the most important and long-standing conjectures in graph theory. Reed and Seymour showed in 2004 that Hadwigers conjecture is true for line graphs. We investigate this conjecture on the closely related class of total g
Motivated by a hat guessing problem proposed by Iwasawa cite{Iwasawa10}, Butler and Graham cite{Butler11} made the following conjecture on the existence of certain way of marking the {em coordinate lines} in $[k]^n$: there exists a way to mark one po
We prove a conjecture of Ohba which says that every graph $G$ on at most $2chi(G)+1$ vertices satisfies $chi_ell(G)=chi(G)$.
We consider three graphs, $G_{7,3}$, $G_{7,4}$, and $G_{7,6}$, related to Kellers conjecture in dimension 7. The conjecture is false for this dimension if and only if at least one of the graphs contains a clique of size $2^7 = 128$. We present an aut