Conjecture on bichromatic triangles or quadrangles around a blue polygon
Conjecture on bichromatic triangles or quadrangles around a blue polygon
Consider a bicolored pseudoline arrangement, with pseudolines colored blue or red. Let be a blue -gon, meaning a polygonal face or region bounded by blue pseudolines, and suppose that at least red pseudolines pass through . A bichromatic triangle or quadrangle on the inside boundary is a triangular or quadrilateral face incident with the inside boundary of and supported by pseudolines of both colors.
Blue-polygon bichromatic-face conjecture. Given a blue -gon with at least 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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