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Criticality, universality, and isoradiality

Critical points and singularities are encountered in the study of critical phenomena in probability and physics. We present recent results concerning the values of such critical points and the nature of the singularities for two prominent probabilistic models, namely percolation and the more general random-cluster model. The main topic is the statement and proof of the criticality and universality of the canonical measure of bond percolation on isoradial graphs (due to the author and Ioan Manolescu). The key technique used in this work is the star--triangle transformation, known also as the Yang--Baxter equation. The second topic reported here is the identification of the critical point of the random-cluster model on the square lattice (due to Beffara and Duminil-Copin), and of the criticality of the canonical measure of the random-cluster model with q \ge 4 on periodic isoradial graphs (by the same authors with Smirnov). The proof of universality for percolation is expected to extend to the random-cluster model on isoradial graphs.