P.N. NATARAJAN, formerly with the Department of Mathematics, Ramakrishna Mission Vivekananda College, Chennai, India, has been an independent researcher and mathematician since his retirement in 2004. He did his Ph.D. at the University of Madras, under Prof. M.S. Rangachari, former director and head of the Ramanujan Institute for Advanced Study in Mathematics, University of Madras. An active researcher, Prof. Natarajan has published over 100 research papers in several international journals like the Proceedings of the American Mathematical Society, Bulletin of the London Mathematical Society, Indagationes Mathematicae, Annales Mathematiques Blaise Pascal, and Commentationes Mathematicae (Prace Matematyczne). His research interests include summability theory and functional analysis (both classical and ultrametric). Professor Natarajan was honored with the Dr. Radhakrishnan Award for the Best Teacher in Mathematics for the year 1990–91 by the Government of Tamil Nadu. In addition to being invited to visit several renowned institutes in Canada, France, Holland and Greece Prof. Natarajan has participated in several international conferences and chaired sessions. He has authored two books, An Introduction to Ultrametric Summability Theory and its second edition, both published with Springer in 2013 and 2015, respectively.