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Abstract

We prove that if a (th r)-convex domain in the hyperbolic plane is covered by n ge 2 circular discs of radius r, then the density of the covering is larger than 2pgr/ \sqrt{27}. The density bound is optimal, and the condition of (th r)-convexity is essentially optimal. Combining our result with earlier estimates yields that if at least two non-overlapping equal circular discs cover a given circular disc in a surface of constant curvature, then the density of the covering is larger than 2pgr/ \sqrt{27}.

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