tailieunhanh - Báo cáo toán học: "On Coloring the Odd-Distance Graph"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: On Coloring the Odd-Distance Graph. | On Coloring the Odd-Distance Graph Jacob Steinhardt j steinha@ Submitted Feb 16 2009 Accepted Apr 14 2009 Published Apr 30 2009 Mathematics Subject Classification 05C15 Abstract We present a proof using spectral techniques that there is no finite measurable coloring of the odd-distance graph. 1 Introduction Let O be the graph with V O R2 and where two vertices are connected if they are at an odd distance from each other. We call O the odd-distance graph. Let the measurable chromatic number of a graph denote the least number of colors needed to color a graph such that each color class is measurable. We aim to show that the measurable chromatic number X of O is infinite. We will do this by defining a sequence of operators Ba related to the adjacency operator of O. We then use an extension of the well-known spectral inequality x G 1 ụ to the infinite-dimensional case. We next determine the set 4min of eigenfunctions for Ba they turn out to be the characters of R2 though this is in a sense guaranteed from the Fourier analysis on R2 . This gives us the full set of eigenvalues for the Ba which we then bound below in order to show that 1 4max goes to X as a goes 1 min to 1. As this is a lower bound for x O we will then have established that O has infinite measurable chromatic number. Throughout the paper whenever we refer to chromatic number we will always mean measurable chromatic number. This result has been proven elsewhere for example as a consequence of the theorem that all measurable sets with positive density at infinity contain all sufficiently large distances see . 2 . However the proof in this paper uses primarily techniques from spectral graph theory rather than measure theory and seems to be closer in spirit to the problem itself. See also 1 where a similar generalization of the Lovasz theta function is studied and used to derive new bounds on the chromatic number of unit-distance graphs in Rn. Section 2 generalizes Hoffman s eigenvalue bound see .

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