Recorded 27 March 2024. Marcin Lis of Technische Universität Wien presents "On Pfaffians in spin models" at IPAM's Statistical Mechanics and Discrete Geometry Workshop. Abstract: It is a classical result of Groeneveld, Boel and Kasteleyn that boundary spin correlations functions in Ising models on planar graphs satisfy Pfaffian relations. Here we consider the reverse question, and show that any classical ferromagnetic spin model whose correlation functions satisfy Pfaffian relations must by (up to local simplifications of the graph) an Ising model on a planar graph. Joint work with Diederik van Engelenburg. Learn more online at: https://www.ipam.ucla.edu/programs/workshops/workshop-i-statistical-mechanics-and-discrete-geometry/