D. J. A. Welsh coauthored a study of square-lattice bond percolation with P. D. Seymour. Their crossing-probability arguments relate crossings of squares to crossings of rectangles. These estimates help control connectivity across different scales and form part of Russo-Seymour-Welsh theorem.[ref]
Percolation probabilities on the square lattice by P. D. Seymour and D. J. A. Welsh. Introduction and Sections 4–5
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