A. Citkin studies for many years the theory of the Yankov characteristic formulas, their generalizations and applications in different areas of propositional, modal and algebraic logic. I. Vandoulakis is Adjunct Professor in history and philosophy of science at the Hellenic Open University, Greece. He holds a Ph.D. in history of mathematics from Moscow M.V. Lomonosov State University (1991). He has published on history and philosophy of Greek mathematics, history of mathematical logic, philosophy and the foundations of mathematics.