CHAPTER 1 Introduction and Results The aim of this paper is twofold: the first, and main one, is to establish a rather satisfying regularity theory for general parabolic systems with degenerate diffusion the second one, of more technical and specialized character, is to introduce a suitable analog of the classical harmonic approximation methods of DeGiorgi’s pioneering work [14] from the elliptic setting, which in turn is the key to a regularity theory for degenerate parabolic systems. 1.1. A short introduction to the regularity of parabolic systems A brief description of the present status of the regularity theory for general parabolic systems cannot begin but with the observation that already in the case of elliptic systems the so called partial regularity – also called almost everywhere regularity – is in general the best one can usually expect, and therefore the same happens in the case of parabolic systems. Indeed, since the important counterex- ample of DeGiorgi [15] – see also [50, 51, 59] – it is known that when dealing with general elliptic systems of the type div a(Du) = 0 or div(A(x)Du) = 0 considered in an open subset Ω ⊆ Rn, solutions might possess singularities, and therefore everywhere regularity fails to hold in general. Instead, one can show partial regularity of solutions, i.e. they are regular outside a negligible closed subset, thereby called the singular set of the solution: (1.1) u ∈ C1,α(Ω loc u , RN) and |Ω \ Ωu| = 0 and we refer to [33, 35, 48] for an account of the theory and a list of references. Eventually estimates for the Hausdorff dimension and boundary regularity can be inferred [46, 47, 24]. Let us mention that related results for integral functionals in the calculus of variations are obtained in [40, 41]. The above partial regularity results for elliptic systems have been extended to the case of parabolic systems of the type (1.2) ∂tu = div a(Du) and we refer for instance to [6, 8, 27, 29, 53, 54] for the most recent and sharp theorems on the issue. Let us meanwhile remark that the system in (1.2), as all the other parabolic ones in this paper, will be considered in the cylindrical domain ΩT := Ω × (0,T ) , where Ω ⊆ Rn is an open bounded domain with n ≥ 2. Such partial regularity results are however obtained under a non-degenerate ellipticity assumption, both in the case of systems and in that of variational integrals, and this amounts to 1

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