Dr Pedro Calderón had a natural affinity for mathematics and music. He would have undoubtedly subscribed to Leibniz's famous saying that: "Music is the secret arithmetic of the soul, unaware of its act of counting." The fact is that he tried to instil in his sons, at an early age, a keen interest in mathematics and music. "At the dinner table he would challenge Alberto, a boy of six or seven, to make rapid mental calculations; or he would play classical music for Alberto an his older sister Nenacha."

With the years, I would take Complex Analysis, Ordinary Differential Equations, Funciones Reales II, Partial Differential Equations, Topology, Functional Analysis, Projective Geometry, Differential Geometry, and other important subjects. We had a first rate faculty teaching these fundamental courses. Among them I remember: Alberto González Domínguez, Mischa Cotlar, Luis Santaló, Mario Gutiérrez Burzaco, Rafael Panzone, Agnes Benedek, Miguel Herrera, and Prof Roque Carranza (statistician). This period was replete of personal experiences. I met mathematicians like Dr Julio Rey Pastor and Beppo Levi during the mathematical teas that were held in the OEA building of Avenida de Mayo. A full floor had been loaned to the School of Exact Sciences of the University of Buenos Aires.

Shortly before my graduation as "Licenciado en Ciencias Matemáticas," I took a course on "Boundary Values of Analytic Functions" that was taught by Dr González Domínguez. That course was an introduction to "mes premiéres armes" in mathematics. Dr González Domínguez would become my thesis adviser later. I learned from him topics such as the Central Limit Theorem in Probability Theory, Introduction to Functions of Hille (Hermite Expansions), Laguerre Expansions, Theory of Approximation and Theory of Distributions. ... the most discernible influence on González Domínguez was Einer Hille's work "A Class of Reciprocal Functions." That influence was passed on to me and from me on to some of my own students. During my formative years, Dr Rafael Panzone and his wife Dr Agnes Benedek had a tremendous influence in developing my mathematical taste and knowledge.

On this occasion, I want to thank Dr Alberto González Domínguez, who proposed the problem to me, for his assistance during the preparation of the work and for his opportune suggestions to improve some proofs.

At that time I met Eugene Fabes, who would have a great influence on my career. I expanded my professional interests as to include Singular Integrals, Differentiation Theory and, much later in time, Differential Equations. This was the time when I became acquainted with the Navier Stokes Equations.

... Calixto became my mentor. Beyond mathematics, Calixto taught us how to be an analyst, to seek the heart of an issue, to understand a problem. In fact, Calixto is a non-conventional mathematician and his work has expended in diverse areas, and presented new insights. By that time Calixto had already started to think on new issues concerning harmonic analysis and partial differential equations.

I came to the United States to the University of Minnesota in January of 1973. There was no email, no fax, and even the telephone was very expensive. But I found at Minnesota and in the mid-west an extraordinary group of people. My colleagues were extremely generous, dedicated, and friendly, and they taught me much of what I know. They shared their ideas and gave me guidance as I began my research programme.

Once more, he embedded himself in new problems and ideas, some connected to his past research, others bold and new. From these times are his articles on existence of solutions to Navier Stokes Equations in a very particular norm and through the years Calixto published articles in diverse areas such as tumour growth, multiscale species competition, probability. For Calixto, doing mathematics never felt like an obligation but always an enjoyment. He also took a strong interest at this time in the history of science and, in particular, the early contributions to science of the Hispanic culture.

Aside from his primary areas of interest in harmonic analysis, it is important to mention that at some point in the career of Prof Calderón, he started getting interested in the applications of mathematical ideas and techniques to problems of biology and medicine. It had always been the view of Prof Calderón that biology and medicine could benefit from systematised mathematical support just as how the disciplines of physics and engineering benefitted immensely from such support many years ago. It is, therefore, not surprising that a number of doctoral candidates produced by Prof Calderón completed their dissertations on topics related to mathematical biology.

After my retirement. I remained in Chicago. I contacted the analysts at De Paul University ... I had the chance to participate actively in their Analysis Seminar.

Calixto P Calderón, like many Argentine mathematicians of his generation, was introduced and mentored into Harmonic Analysis and Partial Differential Equations at the University of Buenos Aires by Alberto González Domínguez, his teacher and Ph.D. advisor, whose mentor was David Tamarkin of Brown University. Moreover, Calixto P Calderón proved very early in his career to be an influential teacher and mentor for Luis A Caffarelli, a world authority on Free Boundary Value Problems in Partial Differential Equations and recent recipient of the prestigious Israel's Wolf Prize.

The papers presented by his friends or colleagues and some of his former doctoral students at the conference in honour of Calixto P Calderón at Roosevelt University were indicative to a large extent of the wide scope and depth of his research work and scholarship in pure and applied mathematics. Furthermore, the friendly and festive atmosphere among the participants at the conference from the USA and Latin America reflected very much his wonderful charismatic personality and humility. A memorable moment was the scene where the president of Roosevelt University, while addressing the participants briefly, asked those in the audience who were Calixto P Calderón's students to raise their hands, and Luis A Caffarelli's hand went up!

Calixto P Calderón's numerous research publications include: Classical Fourier series and Orthogonal Expansions, Calderón-Zygmund Theory of Singular Integrals, Ordinary and Partial Differential Equations especially the Navier-Stokes p.d.e, Probability Theory, Mathematical Models Applied to Biology and Medicine, and History of Mathematics and Science. Clearly, his work in pure mathematics was influenced by Alberto González Domínguez, Alberto P Calderón - his brother, Antoni Zygmund, and several other collaborators and colleagues. Whereas his considerable research work in Mathematical Biology and Medicine might have been the result of the influence he received while growing up from his father, Dr Pedro J Calderón, an accomplished physician trained in Medicine in Buenos Aires and Paris. One can't help but note that, likewise, the father of Aristotle, Nicomachus, who did experimental research in Biology and Botany, although he was Plato's greatest student of pure philosophy, was also a medical physician and, in fact, the personal physician of King Philip of Macedon.

Except for the appointments at the University of Cuyo, the University of Buenos Aires, and the University of Minnesota, Calixto P Calderón spent most of his distinguished academic career at the University of Illinois at Chicago (U.I.C.). He taught a variety of courses ranging from Harmonic Analysis and Partial Differential Equations to Probability Theory and Statistics, and had several Ph.D. students. His strong interest in probability and statistics may be attributed in part to his Statistics teacher - Roque Carranza at the University of Buenos Aires. He wrote two papers on the subject including a paper on Kolmogorov's strong law of large numbers for pair-wise independent random variables. The themes of the thesis of his Ph.D. students, like his research, ranged from Harmonic Analysis to the history of Spanish mathematics and related science. During his tenure at U.I.C., Calixto P Calderón was an active participant and invited speaker at seminars in the Chicago area, including the Calderón-Zygmund Seminar at the University of Chicago, and the analysis seminars at U.I.C., and at DePaul University. Furthermore, Calixto P Calderón was an excellent and eloquent lecturer, and remained actively engaged in issues related to mathematics education at U.I.C.