Suppose that X is a bounded-degree polynomial with nonnegative coefficients on the p-biased discrete hypercube. Our main result gives sharp estimates on the logarithmic upper tail probability of X whenever an associated extremal problem satisfies a certain entropic stability property. We apply this result to solve two long-standing open problems in probabilistic combinatorics: the upper tail problem for the number of arithmetic progressions of a fixed length in the p-random subset of the integers and the upper tail problem for the number of cliques of a fixed size in the random graph Gn,p. We also make significant progress on the upper tail problem for the number of copies of a fixed regular graph H in Gn,p. To accommodate readers who are interested in learning the basic method, we include a short, self-contained solution to the upper tail problem for the number of triangles in Gn,p for all p = p(n) satisfying n −1 log n p 1. Contents 42 8. The Poisson regime 50 9. Beyond polynomials with nonnegative coefficients 58 10. Concluding remarks 59 References 61

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In this article, we prove that the height function associated with the square-ice model (i.e. the six-vertex model with a = b = c = 1 on the square lattice), or, equivalently, of the uniform random homomorphisms from Z 2 to Z, has logarithmic variance. This establishes a strong form of roughness of this height function.

Let G be a quasi-transitive graph on V. A random field X = (Xv) v∈V whose distribution is invariant under all automorphisms of G is said to be a factor of i.i.d. if there exists an i.i.d. process Y = (Yv) v∈V and an equivariant map ϕ such that ϕ(Y ) has the same distribution as X. Such a map, also called a coding, is said to be finitary if, for every v ∈ V, there exists a finite (but random) set U ⊂ V such that ϕ(Y )v is determined by {Yu} u∈U . We construct a coding for the random-cluster model on general quasi-transitive graphs, and show that the coding is finitary whenever the free and wired measures coincide. This strengthens a result of Häggström-Jonasson-Lyons [18]. We also prove that the coding radius has exponential tails in the sub-critical regime. As a corollary, we obtain a finitary coding for the sub-critical Potts model on G whose coding radius has exponential tails. In the case of G = Z d , we also construct a finitary, translation-equivariant coding for the sub-critical random-cluster and Potts models using a finite-valued i.i.d. process Y . To do this, we extend a mixing-time result of to infinite-range monotone models on quasi-transitive graphs of sub-exponential growth. Our methods also apply to any monotone model satisfying mild technical (but natural) requirements.

It is shown that the Villain model of two-component spins over two dimensional lattices exhibits slow, non-summable, decay of correlations at any temperature at which the dual integer-valued Gaussian field exhibits depinning. For the latter, we extend the recent proof by P. Lammers of the existence of a depinning transition in the integervalued Gaussian field in two-dimensional graphs of degree three to all doubly-periodic graphs, in particular to Z 2 . Taken together these two statements yield a new perspective on the Berezinskii-Kosterlitz-Thouless phase transition in the Villain model, and complete a new proof of depinning in two-dimensional integer-valued height functions.