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(Almost) Impossible Integrals, Sums, and Series with Foreword by Paul J Problem Books in Mathematics Cornel Ioan Vălean (Almost) Impossible Integrals, Sums, and Series With foreword by Paul J. Nahin Problem Books in Mathematics Series Editor: Peter Winkler Department of Mathematics Dartmouth College Hanover, NH USA More information about this series at http://www.springer.com/series/714 Cornel Ioan Valean˘ (Almost) Impossible Integrals, Sums, and Series With a Foreword by Paul J. Nahin 123 Cornel Ioan Valean˘ Teremia Mare, Timis, County Romania ISSN 0941-3502 ISSN 2197-8506 (electronic) Problem Books in Mathematics ISBN 978-3-030-02461-1 ISBN 978-3-030-02462-8 (eBook) https://doi.org/10.1007/978-3-030-02462-8 Library of Congress Control Number: 2018966810 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland To my forever living parents, Ileana Ursachi and Ionel Valean Foreword Question: “Can you tell me who can help me do some elliptic integrals?” Answer: “We’ve tried to get rid of anyone like that.” —Exchange between a physics graduate student and a professor of mathematics1 Shortly after my book Inside Interesting Integrals was published by Springer in August 2014, I began to receive e-mails from all over the world. They were from readers who were writing to show me how to do one or the other of the problems in my book in a way “easier,” or “more direct,” than was the solution I gave. Almost all were fascinating reading, and those communications confirmed my belief that people who buy math books are quite different from those who don’t. One of those correspondents was the author of the book you now hold in your hands. Cornel, in fact, wrote to me (from his home in Romania) numerous times over the following months. One of his first e-mails was to take exception to my claim that a result attributed to the great Cauchy, himself, ∞ ecos(x) sin {sin (x)} π dx = (e − 1) , 0 x 2 would be “pretty darn tough [to do]” by means other than contour integration (which is how I do it in my book). Cornel’s clever solution, however, using just “routine methods” available to any undergraduate in math, physics, or engineering by the end of their second year, is indeed much easier, and later in this book, you’ll see just how he does it. I was impressed, yes, but soon put it aside and turned to other matters. 1From an e-mail I received, after the publication of Inside Interesting Integrals, from Lawrence Glasser, Professor Emeritus of Physics at Clarkson University, Potsdam, New York, USA. Larry was the grad student. I’ll explain the significance of this quote by the end of this essay. vii viii Foreword Later communications from Cornel, however, increased my interest. But what really convinced me that Cornel wasn’t “just” a clever math aficionado but rather is a seriously talented mathematician was when he sent me the calculation of 1 1 1 x y z n dx dy dz 0 0 0 y z x where {x} denotes the fractional part of x and the integer n ≥ 1. I had never seen anything like it before. It is, of course, immediately clear that the integral exists, as the integrand is always in the interval 0 to 1 over the entire finite region of integration (the volume of the unit cube). But how to do the triple integration completely baffled me. And then, turning to the final page of Cornel’s five-page derivation that he had mercifully included in his e-mail (else I would have gone mad with frustration), his answer was just too wonderful to simply be made-up: using his general result that is a function of n, he gave the explicit solutions for the first five values of n, the first two of which I’ll repeat here: 1 1 1 x y z 3 1 dx dy dz = 1 − ζ (2) + ζ (3) ζ (2) 0 0 0 y z x 4 6 and 1 1 1 x y z 2 1 1 7 dx dy dz = 1 − ζ (2) − ζ (3) + ζ (6) 0 0 0 y z x 2 2 48 1 1 + ζ (3) ζ (2) + ζ 2 (3) 18 18 1 + ζ (3) ζ (4) , 12 where ζ denotes the famous Euler–Riemann zeta function, well-known to physicists and mathematicians alike. You’ll see later in this book how he arrived at these amazing results (wait until you see the n = 5 case!), but one thing I could immediately do was a computer check. The first two theoretical results on the right-hand side of the equality signs are: (n = 1) 0.095850 ... and (n = 2) 0.023409 ... while a direct numerical evaluation of the integrals gives the values of (n = 1) 0.095844 ... and (n = 2) 0.023335 ... Foreword ix Here we have an agreement between Cornel’s theory and numerical calculations out to “only” three or four decimal places, but, when you consider how wild2 are the fluctuations of the integrand as we move from point to point in the unit cube, what impresses me is that we have even that much agreement. As I looked at these results, I knew I had a correspondent of real talent. My curiosity now fully engaged, I plunged into Cornel’s detailed derivation of the triple integral, and, for page after page, it was just one incredibly clever, occasionally devious, sly trick after another. I knew, as I staggered from one line to the next (often after scribbling away for half an hour or more before I caught on to what he was doing), that I was following the path of the most creative person. You’ll find that this entire book is like that, with one spectacular computation after another. I predict you’ll have a hard time in putting this book down once you start. You’ll find results in here that you have never seen before or, if you have, with an ingenious derivation that you haven’t seen before. I predict, if you love mathematics, that you are in for a great time. As you must surely now be wondering, just as I did, who is Cornel Ioan Valean?˘ Not being a particularly subtle person, I simply asked him. Cornel revealed to me that he was 37 years old in 2015, holds a degree in financial accounting, and, while he has authored or co-authored several papers in various European math journals (including The Gazette of the Royal Spanish Mathematical Society and the Journal of Classical Analysis), he is not a professional mathematician in an academic post. Rather, in the words of the great French mathematician Henri Poincaré (1854– 1912), he is a person blessed with a “special intuition [that allows someone] to perceive at a glance” the solution to a problem.3 Today, alas, more than a century later, not much more than that is known about how the mathematical mind works. In Cornel’s own words about this, words he wrote to me when I asked him specifically about how he works, “I have many ideas for research (I don’t know where they come from but it’s like a flood), maybe it’s simply the natural expression of a crazy passion.” 2 = x If you make a three-dimensional sketch of f y as x and y each vary over the interval 0 to 1, you’ll quickly appreciate why I use the word wild. 3From an address Poincaré gave in 1908, titled “Mathematical Creation,” to the Société de Psychologie in Paris. You can find an English translation reprinted in a collection of Poincaré’s works, The Foundations of Science (translation by G. B. Halstead), The Science Press 1929, pp. 383–394. Some years later the famous French mathematician Jacques Hadamard (1865–1963) tried his hand, too, at answering the puzzle of mathematical creation, with a little book called The Psychology of Invention in the Mathematical Field, Princeton 1945, but I don’t believe he offered any new insights beyond Poincaré’s. x Foreword Encountering someone like Cornel, with what Poincaré called a “special intu- ition,” is an exciting experience. An illustration of such an experience was nicely described4 a few years ago by Tulane University’s mathematics professor Victor Moll, in the story of how his book Irresistible Integrals, written in collaboration with the late George Boros, came to be.5 Moll had actually been aware of Boros long before Boros came to be Moll’s doctoral student at Tulane.
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