The contributions of Lucio Russo to the mathematics of percolation and disordered systems are outlined. The context of his work is explained, and its ongoing impact on current work is described and amplified.
A personal appreciationPrior to his mid-career move to the history of science in the early 1990s, Lucio Russo enjoyed a very successful and influential career in the theory of probability and disordered systems, in particular of percolation and the Ising model. His ideas have shaped these significant fields of science, and his name will always be associated with a number of fundamental techniques of enduring importance.The author of this memoir is proud to have known Lucio in those days, and to have profited from his work, ideas, and company. He hopes that this brief account of some of Lucio's results will stand as testament to the beauty and impact of his ideas.
Scientific summaryLucio Russo has worked principally on the mathematics of percolation, that is, of the existence (or not) of infinite connected clusters within a disordered spatial network. The principal model in this field is the so called percolation model, introduced to mathematicians by Broadbent and Hammersley in 1957, [14]. Consider, for definiteness, the hypercubic lattice ޚ d with d ≥ 2, and let p ∈ [0, 1]. We declare each edge to be open with probability p and closed otherwise, and different edges receive independent states. The main questions are centred around the existence Let C be the open cluster of ޚ d containing the origin. Two functions that play important roles in the theory are the percolation probability θ and the mean cluster size χ given by Lucio contributed a number of fundamental techniques to percolation theory during the period 1978-1988, and the main purpose of the current paper is to describe these and to explore their significance. We mention Russo's formula, the Russo-Seymour-Welsh (RSW) inequalities, his study of percolation surfaces in three dimensions, and of the uniqueness of the infinite open cluster, and finally Russo's approximate zero-one law. Russo's formula and RSW theory have proved of especially lasting value in, for example, recent developments concerning conformal invariance for critical percolation.In Section 8, we mention some of Lucio's results concerning percolation of +/− spins in the two-dimensional Ising model. It was quite a novelty in the 1970s to use percolation as a tool to understand long-range order in the Ising model. Indeed, Lucio's work on the percolation model was motivated in part by his search for rigorous results in statistical mechanics. His approach to the Ising model has been valuable in two dimensions. In more general situations, the correct geometrical model has been recognised since to be the random-cluster model of Fortuin and Kasteleyn (see [26]).This short account is confined to Lucio's contributions to percolation, and does not touch on his work lying closer to ergodic theory and dynamical systems, namely [R2; R6; R8; R11], and neither does it refer to the paper [R7]. A ...