We, the guest editors, are very pleased to dedicate this special issue to two mathematicians, well-known within the realms of mathematical optimization.
Therefore, it is a great honour for us to dedicate this issue to 
Prof. Dr. Dr. h.c. Franco Giannessi in celebration of his 80th birthday and to
Prof. Dr. Dr. h.c. Diethard Pallaschke in celebration of his 75th birthday.
Both mathematicians devoted much time many years to this journal and other journals as associate editors, reviewers and editors-in-chief.
The mathematical works of both scientists whose original ideas influenced various fields in mathematics, such as functional analysis, convex analysis, game theory, variational calculus, geometry and, last but not least, optimization, are quoted by many authors.
What both scientists have as a great deal in common is that their originality comes from their ability to put and answer penetrating questions, and so many co-workers have benefited from their deep mathematical thinking.
It would mean carry coals to Newcastle to list all their scientific papers and books here. Today everybody can read their journey in science by their works on electronic platforms. Please look for instance in MathSciNet to see the huge number of Prof. Giannessi’s and Prof. Pallaschke’s scientific contributions.
Let us give our deep gratitude to both colleagues and friends for their long-standing co-operation over more than 30 years, and let us wish them all the best in the future.
We also give our great thanks to the 27 authors who responded in this special issue to honour both gentle persons and uncompromising fighters for scientific truth.
Juan Enrique Martínez-Legaz
Boris Mordukhovich
Christiane Tammer