Sharp comparison and maximum principles via horizontal normal mapping in the Heisenberg group

Balogh, Zoltan and Calogero, Andrea and Kristály, Alexandru (2015) Sharp comparison and maximum principles via horizontal normal mapping in the Heisenberg group. JOURNAL OF FUNCTIONAL ANALYSIS, 269 (9). pp. 2669-2708. ISSN 0022-1236

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Abstract

In this paper we solve a problem raised by Gutiérrez and Montanari about comparison principles for H-convex functions on subdomains of Heisenberg groups. Our approach is based on the notion of the sub-Riemannian horizontal normal mapping and uses degree theory for set-valued maps. The statement of the comparison principle combined with a Harnack inequality is applied to prove the Aleksandrov-type maximum principle, describing the correct boundary behavior of continuous H-convex functions vanishing at the boundary of horizontally bounded subdomains of Heisenberg groups. This result answers a question by Garofalo and Tournier. The sharpness of our results are illustrated by examples.

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