For any integer k, we prove the existence of a uniquely
k-colourable graph of girth at least
g on at most k12(g+1) vertices whose
maximal degree
is at most 5k13. From this we deduce
that, unless NP=RP, no polynomial time algorithm for k-Colourability
on graphs G of girth
g(G)[ges ]log[mid ]G[mid ]/13logk
and
maximum degree Δ(G)[les ]6k13
can exist. We also study several related problems.
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