Dirichlet: A Mathematical Biography by Uta C. Merzbach

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Dirichlet cannot be surpassed for richness of material and clear insight into it: as a speaker he has no advantages—there is nothing like fluency about him, and yet an eye and understanding make it indispensable: without an effort you would not notice his hesitating speech. What is peculiar in him, he never sees his audience—when he does not use the blackboard at which time his back is turned to us, he sits at the highest desk facing us, puts his spectacles up on his forehead, leans his head on both hands, and keeps his eyes, when not covered with his hands, mostly shut. He uses no notes, inside his hands he sees an imaginary calculation, and reads it out to us— that we understand it as well as if we saw it too. I like that kind of lecturing … Dirichlet also has his peculiarities—one is of forgetting time; he pulls his watch out, finds it past three, and runs out without even finishing the sentence. o wrote the Englishman Thomas Archer Hirst in his extensive diaries [1] during a six-month visit to Berlin in which he befriended Lejeune Dirichlet and his family (see [2]). But in spite of his unusual teaching methods, Dirichlet was, according to Oystein Ore [3], ‘‘an excellent teacher, always expressing himself with great clarity.’’ Nowadays, Dirichlet is mainly remembered for his farreaching research achievements, which made a great impact in the middle of the nineteenth century. These ranged, as we shall discover, from his many contributions to number theory and the study of series to a modern definition of a function and his work in potential theory and stability. As the preface to the book under review asserts:

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Dirichlet was a mathematician who shared responsibility for significant transformations in mathematics during the nineteenth century in Western Europe, a notable period in the history of mathematics. Yet there is no book-length study of his life and work and only a small proportion of his publications tends to be cited. In Chapter 1 we learn that Dirichlet was born in Du¨ren, in the Rhineland, in 1805, while it was still under French occupation before becoming part of Prussia in 1815. His name appears in the register as Jean Pierre Gustave Lejeune de Richelet. He attended local schools, where he became fluent in French, before taking up residence in Bonn at the

age of 12, and then in Cologne three years later, where he was taught mathematics and physics by Georg Simon Ohm (of Ohm’s law fame). From the very beginning, the young Dirichlet had become fascinated by mathematics, but he struggled with Latin, a fact of great significance later on, as we shall see. After Chapter 1, the chapters mainly alternate between those of a biographical nature and those in which Dirichlet’s mathematical achievements are explained in some detail, so that readers who are interested in only one aspect of his life can choose between the even-numbered and the odd-numbered ones. Each of the biographical chapters consists of several short sections that carry the chronological development while digressing into informative descrip