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The Self-Avoiding Walk Connective Constant
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Posed by question implicit in Orrick's 1947 work on polymer chains; formalised by Hammersley · 1954 · probability theory / statistical mechanics · ~1 min read
· difficulty 4/5
The problem
Let \(c_n\) count the number of n-step self-avoiding walks starting at the origin on ℤ². Question: determine the connective constant μ = \(lim_{n→∞}\) \(c_n^{1/n}\). For the hexagonal lattice, μ = √(2+√2) exactly (Duminil-Copin–Smirnov 2010). For the square lattice, μ ≈ 2.638 from numerical estimates but no exact value or rigorous proof.
History & significance
Orrick considered self-avoiding polymer chains in the 1940s; Hammersley and Morton formalised the connective constant and proved submultiplicativity (\(c_{m+n}\) ≤ \(c_m\) · \(c_{n}\)), guaranteeing the limit exists. Nienhuis's 1982 Coulomb gas prediction gave μ ≈ 2.638 for the square lattice via Coulomb-Franck constants. Lawler–Schramm–Werner's SLE framework (2004 Fields work) confirmed fractal dimension predictions but not μ itself.
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