A one-page solution of a problem of Erdős and Purdy (Q2197690)

The following conjecture has been attributed to [\textit{P. Erdős} and \textit{G. Purdy}, J. Comb. Theory, Ser. A 25, 205--210 (1978; Zbl 0422.05023)]: Assume \(P\) is a set of \(n\) points in general position in the plane, where \(n > 4\) is even. Assume \(R\) is another set of points disjoint from \( P\), such that every line through two points of \(P\) contains a point from \(R\). Then \(|R|\geq n\). By now, the conjecture has several proofs. The paper under review provides a short and elementary, one-page proof to the conjecture. This proof may be the ``book proof'' of the result.