The first 20 years of my life were spent in the intensely religious and ritualistic atmosphere of a south Indian Brahmin family confined to the towns of Thanjavur and Madras and the paddy fields of the village of Thalanayar in the Thanjavur district. Emphasis was laid at home on the philosophy and teachings of the Vaishnavite saints Ramanuja and Desika, the Hindu epics 'Ramayana' and 'Mahabharata', the devotional songs of Tamil saints known as the 'Alwars' and finally the 'Bhagavad Gita'. To this day, I believe in their basic idea that personal and global peace are possible only through renunciation and action with detachment.

My father once told me that when he was studying mathematics honours in Vivekananda college in Madras, notebooks were expensive and he had just one notebook. So during the evening, when he returned to his joint family home in Alamelu Manga Puram, he would make calculations with chalk on the floor of his bedroom and after all the calculations were done, he would record the final result in this notebook.

We also had a mathematical club run by Subramaniam where we listened to seminars on varied topics, for example, on the proof that a fifth degree equation doesn't have solutions in radicals. So we had a good time.

In 1956 a young man from Madras reaches the Indian Statistical Institute, Calcutta to continue his studies. He was late by a day to join the course. When he attended the first class, there was an ordinary looking man lecturing and conducting a practical session. When asked if he had brought a practical note book, the young man replied that he did not know he required one and would get it later. The teacher was stern and told him that even if he is late, he should have found out before attending the class. The young man, who did not like the stern warning, went to complain to the director C R Rao. But, on entering his office, he quickly turned back. That ordinary looking teacher was C R Rao and the young man was K R Parthasarathy.

... we didn't have a good hostel. We all lived in sheds that had an asbestos roof and bamboo partitions, but the surroundings were green. There were ponds in which we could swim everyday and enjoy ourselves, but there was always this difficulty in getting clean water. Many students did fall sick. But at ISI, for the first time, we were attending classes where we could listen to proofs of theorems originally proved by our own teachers which was in stark contrast to our college, where we thought that all theorems came from Newton, Euler, Gauss and such people. They had written books from which our courses were taught, and so it was a completely different experience.

In those days, there was no television, no telephone and we were all afraid of stepping out of the ISI campus because B T Road looked quite dangerous. So most of us just lived on the campus and in the nights all of us assembled in a common room with a big radio and listened to the news. Every Saturday we used to listen to the national programme of music and there were also some students who were extraordinarily good in music. ... Later, in the mess, we discussed all good literature so that was how we came to know about Bengali novels. ... The mess served only vegetarian food with rice, sambar and idli, dosa and pesarat and only south Indian tiffin. The north Indian and Bengali students had a difficult time, and the languages spoken were mostly Tamil and Telugu.

The purpose of this paper is to study a class of irreducible Banach space representations of a complex semi-simple Lie group G through an analysis of the associated representations of the Lie algebra $\mathfrak{g}$ of G.

The work described in this paper was done during 1963-65 when the authors were at the Indian Statistical Institute, Calcutta. At an early stage of the work, the last named author [Varadarajan] had the opportunity of detailed and highly stimulating discussions with Dr S R S Varadhan currently of the Courant Institute of Mathematical Sciences, New York. He would like to express his indebtedness to Dr Varadhan for these discussions which determined our entire approach to the whole circle of questions studied in the present work.

Dr Parthasarathy has done significant work on the application of probability theory to locally compact groups. Working in collaboration with K Schmidt, he has derived infinitely divisible projective representations, the Lévy-Khinchin-Araki formula on locally compact groups and limit theorems for uniformly infinitesimal families of positive definite kernels. His work has had a significant impact in the area of theoretical quantum mechanics associated with names of Araki and Woods.

The basic integrator processes of quantum stochastic calculus, namely, creation, conservation, and annihilation, are introduced in the Hilbert space of square integrable Brownian functionals. Stochastic integrals with respect to these processes and a quantum Itô's formula are described. As an application two examples of quantum stochastic differential equations are discussed. A continuous time version of Stinespring's theorem on completely positive maps in C*-algebras is exploited to formulate the notion of a quantum Markov process and indicate how classical Markov processes are woven into the fabric of the Schrödinger-Heisenberg dynamics of quantum theory.

K R Parthasarathy, Indian Statistical Institute, New Delhi, India, for his significant contributions to both classical and quantum probability and in particular for the development of the quantum stochastic calculus and the unravelling of deep connections that exist between stochastic processes, free quantum fields, group representations and operator algebras.

KRP has always been a no-nonsense man, very demanding in mathematical expositions. His favourite line was "writing or talking mathematics is all about economy of thought and of expression." He is blessed with the extraordinary facility of jumping between different areas of mathematics - in a sense he has the ability to see connections where most others would not. The areas of KRP's research is too vast to describe here in any detail. The canvas encompasses: (i) Information theory, classical in early years and quantum in the last decade; (ii) Limit theorems, weak convergence and infinite divisibility in probability theory; (iii) Lie groups and probability measures on them; (iv) Mathematical formulations of quantum mechanics - imprimitivity systems and perturbation of operators in a Hilbert space; (v) Quantum stochastic calculus - in which he was the pioneer along with R L Hudson.

KRP is a brilliant teacher and an inspiration to many generations of Indian students and researchers. He has authored many books, monographs and lecture-notes, all written in his impeccable style, and they are treasured by the students and researchers world over.

Professor Parthasarathy is one of the most distinguished mathematicians that India has produced in the twentieth century. He has made profound and original contributions to a large number of areas of contemporary mathematics and mathematical physics such as ergodic theory, information theory (classical and quantum), quantum stochastic analysis, coding theory and representation theory of Lie groups (both finite and infinite dimensional).

In the theory of probability, Markov processes and the associated stochastic analysis, created by K Ito, D Stroock, and S R S Varadhan, occupy a central place. K R Parthasarathy and Hudson carried out a far-reaching generalisation of this theory to the non-commutative quantum world, in a series of fundamental papers. It is very likely that this theory will play a key role in future developments of non-commutative geometry and mathematical physics.

Professor Parthasarthy's work is characterised by a directness of approach, simplicity and penetrating power. Especially to be singled out are his papers on representations of Lie groups and gauge groups, and his papers on quantum stochastic processes.

During his long and distinguished career of over a fifty years, he has fulfilled, in an exemplary manner, diverse roles: independent thinker and researcher, inspiring teacher, guide and role model to younger scientists.

KRP is well known for several books and lecture notes. He was a scintillating and inspiring lecturer and his talks were universally admired as models of clear exposition. He remained an enthusiastic learner and teacher till the very end of his life, writing his last paper shortly before his death.

KRP also served on several committees with dedication. To the staid proceedings of their meetings, he often brought colour with his acerbic wit and irreverence. He was a great raconteur and often regaled his students with imitations of P C Mahalanobis and a speech of Russian leader Nikita Khrushchev that he had heard during the Cuban crisis when he was a visitor to the Soviet Union.

In his passing away, we have lost one of India's finest mathematicians, scholars and teachers of the post-Independence era, whose research received high international acclaim and who inspired several generations of students.