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Gruber P. Convex and Discrete Geometry

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512 <strong>Geometry</strong> of Numbers<br />

curves to each of the boundary vertices of G so as to get a new planar graph. This<br />

new graph has only triangular countries. According to (1), construct a corresponding<br />

packing of discs in C. After a suitable Möbius transformation has been applied, we<br />

may assume that the new vertex corresponds to the exterior of D, all other discs form<br />

a packing in D <strong>and</strong> the discs which correspond to the boundary vertices of G touch the<br />

boundary of D (Fig. 34.7). Now define a map fε as follows: fε maps each vertex of<br />

G onto the centre of the corresponding disc. Then extend fε affinely to the triangular<br />

countries of G. This gives a piecewise affine mapping of the union of the countries of<br />

G, excluding the exterior country, into int D. The six circle conjecture of Fejes Tóth<br />

proved by Bárány, Füredi <strong>and</strong> Pach [70], or the hexagonal packing lemma of Rodin<br />

<strong>and</strong> Sullivan [844] proved by He [484], shows that, for small ε>0, triangles deep<br />

in the interior of the graph G are mapped onto almost regular triangles. This, in turn,<br />

shows that the function fε is almost analytic deep in the interior of J.<br />

As ε →+0, the functions fε (possibly after suitable Möbius transformations are<br />

applied) converge to a Riemann mapping function f : J → int D.

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