Beraha's conjecture on chromatic roots near the Beraha numbers

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For each integer r≥2r\geq2, define the Beraha number

br=2+2cos⁡2πr.b_r=2+2\cos\frac{2\pi}{r}.

A plane triangulation is a loopless plane graph whose faces all have size three. Beraha's conjecture. For every r≥2r\geq2 and every ϵ>0\epsilon>0, there exists a plane triangulation with a real chromatic root in

(br−ϵ,br+ϵ).(b_r-\epsilon,b_r+\epsilon).

This would imply the existence of plane triangulations with real chromatic roots arbitrarily close to 44, since brb_r tends to 44. The source says that this remains open, while complex roots arbitrarily close to 44 are known.

References

Primary source

Bill Jackson, “Zeros of Chromatic and Flow Polynomials of Graphs”, arXiv:math/0205047 (2002).

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