OBITUARY

NICOLAAS DU PLESSIS

Nicolaas du Plessis was born in Johannesburg, South Africa, on 2 April 1921, theonly child of Aletta and Barend Johannes Andries du Plessis, a time-keeper for theTransvaal Chamber of Mines. He was educated at Pretoria Boys' High School wherein 1937 he gained a first class matriculation pass. A loan from the South Africangovernment enabled him to go to the University of the Witwatersrand (Wits) as anEducation student. He majored in Pure Mathematics and Physics, gained a FirstClass Honours degree and distinguished himself, being awarded the William Cullenmedal as the best science graduate of the year.

Because of his obligation to teach to repay the government loan, he was notimmediately able to pursue his mathematical studies but had a year of teachertraining. During this time he read G. H. Hardy's Pure mathematics, and thus wasborn his ambition to go to Cambridge. However, the war meant that such plans hadto be abandoned for the duration. Although as a graduate Nick could have taken acommission, it was against his principles to take advantage of such a privilege. Heentered the South African Signals, ending the war with the rank of sergeant.Fortunately, for most of the time he was posted to the Bernard Price Institute whichwas on campus at Wits, and was thus able to devote his spare time to his reading andresearch. This did not completely absorb his efforts however for in 1943 Nickmarried Grace Conyers who had been a fellow student on the Science Educationcourse. Wherever one believes good marriages are made, Grace and Nick's wascertainly made there. To those of us who knew them in later years their mutualregard and fulfilment was striking.

On demobilisation in 1945 Nick was employed as a temporary lecturer at Witsand worked on his MSc dissertation. He was awarded the degree (with Distinction)in 1947 and took up a lectureship at the University of Natal in Durban, underProfessor Philip Stein. Two children, Jennifer (1947) and Andrew (1949), were bornduring this time in Natal. In 1949 came Nick's opportunity: he was awarded theJ. B. Webb Gift Research Scholarship. Transport was almost back to normal afterthe chaotic post-war period, but the University would not give him leave of absence.So he resigned, packed up his goods and chattels and set off for Cambridge andLittlewood.

Professor J. E. Littlewood had retired and had no interest in taking on anothercolonial PhD student, but Nick would have no other tutor and finally Littlewood'sresistance was overcome by Nick's importunity. Littlewood took some time to realisethe independence of this young man he had taken on, for Nick had directed his ownstudies for so long, and he had his own ideas on what research he wanted to do. Herejected Littlewood's list of problems, followed his own ideas and asked nothingmore than the opportunity for discussion with his supervisor. In the end he wasallowed to present his thesis in two years—and handwritten at that!

Nick applied for and took up a senior lectureship at Rhodes University,Grahamstown, in 1951. However, his two years in Cambridge had made him veryaware of the mathematical isolation at Rhodes and when the time came for his

Received 3 October, 1983.

Bull. London Math. Soc; 16 (1984), 310-312

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sabbatical leave he determined to take it in Cambridge. The political situation inSouth Africa seemed to him to be becoming more explosive year by year, and thisprovided another reason for hoping to stay in England, although it would meanbreaking ties with so much that was familiar and loved. In 1957 he became a lecturerat King's College, Newcastle upon Tyne, a post which, being promoted to seniorlecturer in 1963, he held for the remaining 26 years of his life. The new environmentproved congenial, the majority of his research papers being written during his firsteight years in Newcastle. He developed a number of other interests in the Universityboth professionally in the field of teaching mathematics and in the community, beingat various times President of the Staff Christian Group and Methsoc (the UniversityMethodist Society). He ran evening classes to help school teachers understand the"New Mathematics", a problem which had gone unnoticed by many of theexponents of the change. His interest in the teaching of mathematics was of greathelp to the department and its students during the rapid expansion of the 1960swhen many lectureships were held by young research-orientated PhDs, who oftenstayed no more than a year or two and had rather sketchy ideas about what anaverage student could be expected to absorb. Nick was able to supply the experiencewhich so many of us lacked. His interest in providing courses in which the needs ofthe less able students were not ignored developed and, for many years, he wasresponsible for overseeing lower level and "problem" courses in the department.

In the 1960s the direction of Nick's research changed. His earlier papers were inclassical analysis, but in 1962 he spent 6 months in Paris studying modern potentialtheory under Brelot and Choquet. Over the next few years he published three papersin the subject ([10,11,13]) and a book [14]. The book, written at MSc level, was thefirst extensive account of this material in English and served to introduce it toworkers in this country and the rest of the English-speaking world (Nick himself wasan accomplished linguist, as besides English and his native Afrikaans he read French,German and Russian). The book was mainly concerned with the solution of theDirichlet problem for very general domains but, once it had been published, Nickturned to the Neumann problem on which he worked for the rest of his life. Hisinterests in teaching continued unabated. He supervised one PhD student,Dr. B. W. Martin, and gave MSc lectures in partial differential equations. He spenttwo sabbatical years in the USA—1968-69 at Maryland and 1974-75 at Boulder.

The influence of Nick and Grace on their children was a strong one—both readmathematics at Cambridge, Jennifer going on to take a PGCE and Andrew a PhD atLiverpool under the supervision of Professor C. T. C. Wall. Both married and hadchildren. Nick found a new interest: how to devise the most tax efficient means ofinvesting for his grandchildren—a study which also provided some interestingexamples on geometric series for a course in Mathematics for AgriculturalEconomists which he was setting up at the same time. Through the 1970s Nick hadbeen remarkably energetic for his age. The years then began to take their toll, but hisdeath in the early hours of 14 March 1983 was both sudden and totally unexpected.

The author acknowledges the help of a number of people in preparing this note,particularly Mrs Grace du Plessis.

Bibliography1. 'A note about the derivatives of Legendre polynomials', Proc. Amer. Math. Soc, 2 (1951), 950.2. 'A note about functions in Lip a', Proc. Edinburgh Math. Soc. (2), 10 (1954), 100.3. 'The Cesaro summability of Laplace series', J. London Math. Soc, 27 (1952), 337-352.4. 'A theorem about fractional integrals', Proc. Amer. Math. Soc, 3 (1952), 892-898.5. 'Some theorems about the Riesz fractional integral', Trans. Amer. Math. Soc, 80 (1955), 124-134.

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6. 'Spherical fractional integrals', Trans. Amer. Math. Soc, 84 (1957), 262-272.7. 'Half-space analogues of the Fejer-Riesz theorem', J. London Math. Soc, 30 (1955), 296-301.8. 'Spherical Fejer-Riesz theorems', J. London Math. Soc, 31 (1956), 386-391.9. 'Concerning the validity of finite difference operations', J. London Math. Soc, 34 (1959), 208-214.

10. 'Two counter-examples associated with the Dirichlet problem', Quart. J. Math., 15 (1964), 121-130.11. 'Some further examples associated with the Dirichlet problem', Quart. J. Math., 17 (1966), 1-6.12. 'Concerning the validity of finite difference operations (II)', J. London Math. Soc, 41 (1966), 345-352.13. 'Runge's theorem for harmonic functions', J. London Math. Soc. (2), 1 (1969), 404-408.14. An introduction to potential theory (Oliver and Boyd, 1969).

B. E. JOHNSON

School of Mathematics,The University of Newcastle upon Tyne,

Newcastle upon Tyne NE1 7RU.