We prove a Harnack inequality for level sets of $p$-Laplace phase transition minimizers. In particular, if a level set is included in a flat cylinder, then, in the interior, it is included in a flatter one. The extension of a result conjectured by De Giorgi and recently proven by the third author for $p=2$ follows.
Introduction
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Modifications of the potential and of one-dimensional solutions
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Geometry of the touching points
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Measure theoretic results
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Estimates on the measure of the projection of the contact set
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Proof of Theorem 1.1
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Proof of Theorem 1.2
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Proof of Theorem 1.3
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Proof of Theorem 1.4
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Appendix A. Proof of the measure theoretic results