Conjecture on bichromatic triangles or quadrangles around a blue polygon

From papers

Consider a bicolored pseudoline arrangement, with pseudolines colored blue or red. Let PP be a blue kk-gon, meaning a polygonal face or region bounded by blue pseudolines, and suppose that at least k4k-4 red pseudolines pass through PP. A bichromatic triangle or quadrangle on the inside boundary is a triangular or quadrilateral face incident with the inside boundary of PP and supported by pseudolines of both colors.

Blue-polygon bichromatic-face conjecture. Given a blue kk-gon PP with at least k4k-4 red pseudolines passing through, there exists a bichromatic triangle or quadrangle on its inside boundary.

The authors introduce this as an intermediate statement toward proving that every non-trivially bicolored arrangement contains a bichromatic triangle or quadrangle. They report that they could not prove it; its general status is open.

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Sources & referencesView supporting material

Primary source

Yan Alves Radtke, Balázs Keszegh and Robert Lauff, “On Triangles in Colored Pseudoline Arrangements”, arXiv:2601.20574 (2026).

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