Hansjörg Kielhöfer is a Professor at the University of Augsburg, Germany.
This is a friendly and well-written introduction to a topic of great interest in modern analysis. (M. Kunzinger, Monatshefte fur Mathematik, Vol. 193 (4), 2020) This book is an introductory textbook ... . The textbook is appropriate for students with solid background in calculus and linear algebra but without preliminary knowledge of variational analysis. ... this textbook is useful for beginning graduate students in physical, engineering, and mathematical sciences having a rigorous theoretical background. (Mihail Voicu, zbMATH 1390.49001, 2018)