Archive for Princeton University Press

guesstimation (1+2)

Posted in Books, Statistics with tags , , , , , , , , , on November 9, 2012 by xi'an

I received very recently this book, Guesstimation 2.0, written by Lawrence Weinstein from Princeton University Press for review in CHANCE and decided to check the first (2008 )volume, Guesstimation, co-written by Lawrence Weinstein and John A. Adam. (Discovering in the process that they both had a daughter named Rachel, like my daughter!)

The title may be deemed to be very misleading for (unsuspecting) statisticians as, on the one hand, the book does not deal at all with estimation in our sense but with approximation to the right order of magnitude of an unknown quantity. It is thus closer to Innumeracy than to Statistics for Dummies, in that it tries to induce people to take the extra step of evaluating, even roughly, numerical amounts (rather than shying away from it or, worse, of trusting the experts!). For instance, how much area could we cover with the pizza boxes Americans use every year? About the area of New York City. (On the other hand, because Guesstimation forces the reader to quantify one’s guesses about a certain quantity, it has a flavour of prior elicitation and thus this guesstimation could well pass for prior estimation!)

In about 80 questions, Lawrence Weinstein [with John A. Adam in Guesstimation] explains how to roughly “estimate”, i.e. guess, quantities that seem beyond a layman’s reach. Not all questions are interesting, in fact I would argue they are mostly uninteresting per se (e.g., what is the surface of toilet paper used in the U.S.A. over one year? how much could a 1km meteorite impacting the Earth change the length of the day? How many cosmic rays would have passed through a 30 million-year-old bacterium?), as well as very much centred on U.S. idiosyncrasies (i.e., money, food, cars, and cataclysms), and some clearly require more background in physics or mechanics than you could expect from the layman (e.g., the energy of the Sun or of a photon, P=mgh/t, L=mvr (angular momentum), neutrino enery depletion, microwave wavelength, etc. At least the book does not shy away from formulas!) So Guesstimation and Guesstimation 2.0 do not make for a good bedtime read or even for a pleasant linear read. Except between two metro stations. Or when flying to Des Moines next to a drunk woman… However, they provide a large source of diverse examples useful when you teach your kids about sizes and magnitudes (it took me years to convince Rachel that 1 cubic meter was the same as 1000 liters!, she now keeps a post-it over her desk with this equation!), your students about quick and dirty computing, or anyone about their ability to look critically at figures provided in the newsy, the local journal, or the global politician. Or when you suddenly wonder about the energy produced by a Sun made of… gerbils! (This is Problem 8.5 in Guesstimation and the answer is as mind-boggling as the question!) Continue reading

irrationals [guest post/book review]

Posted in Books, University life with tags , , , , , , , , , on June 6, 2012 by xi'an

When I received The irrationals: A story of the numbers you can’t count on by Julian Havil for reviewing for CHANCE, Pierre Alquier happened to be in my office at CREST and I proposed him to write the review, which he did within a few weeks (and thus prior to the book publication!). Here is his nice and comprehensive review:

This book is intended to be a short history of irrational numbers, since the discovery of the first irrational, √3, by the ancient Greeks until the first rigorous definitions of real numbers by Cantor and Dedekind. In addition to the historical aspect, the author does not hesitate to go into mathematical details and to provide some of the most remarkable proofs in the history of irrationals.

The book is essentially organized around the emergence of key mathematical concepts, rather than based on a strict chronological order. Thanks to the historical perspective, we learn a lot about some famous mathematicians like Pythagoras, Euclid, Gauss or Euler. The book is also full of amazing anecdotes. For example, it reveals the way to find the tomb of Roger Apéry, who proved that ζ(3) is irrational, in the labyrinth of Père Lachaise cemetery in Paris. All of this make the reading of this book a real enjoyment. The appendix contains more involved mathematical developments. The only weak point that I would like to point out is the absence of bibliography that would allow the interested reader to go further into the history of number theory, or into number theory itself.

The book can roughly be divided into 4 parts: (1) the discovery of irrationals and the first calculus with square roots, in chapters 1 and 2, (2) the proof that some remarkable numbers like π and e are irrationals in Chapters 3, 4 and 5, (3) some classification of the irrationals based on approximations by rationals, and the discovery of transcendental numbers (Chapters 6, 7 and 8) and, finally, (4) the proper definition of the real numbers by several mathematicians, including Dedekind (9 and 10).

Chapters 1 and 2 deal with the antique world: the proof of the irrationality of √3, the influence of the Pythagoras and Euclid, and the first algebraic manipulations of the irrationals by the Arabs, the Hindus, and European mathematicians like Fibonacci in the early Renaissance. A lot of information is provided about several Greeks mathematicians and philosophers and the reader might sometimes get lost. However, both chapters contain valuable historical information, as well as some nice proofs based on geometry.

Chapters 3, 4 and 5 give the proof of the irrationality of some remarkable numbers. The method of continued fractions is explained in Chapter 3, leading to the irrationality of e. A simpler proof due to Fourier is given in Chapter 4. The proof of the irrationality of π2 (and thus of π) by Hermite is also given in details in that Chapter. Chapter 5 takes the reader to the seventies: it provides the striking proof of that ζ(3) is irrational by Roger Apéry. Surprisingly enough, unlike most recent mathematical proofs, this one only requires a knowledge of elementary mathematics to be understood.

Chapter 6 is one of the most remarkable parts of the book, because of the number of results given there, and the elegance of the proofs. It focuses on approximations of irrationals by rationals. It is obvious that, given any number x and an integer q, one can find another integer p with |x-p/q|<1/q. However, is it possible to find infinitely many p and q such that |x-p/q|<1/q1+ε for a given ε>0? One of the striking facts proved in this chapter is that for ε=1, the answer is yes if, and only if, x is irrational. In Chapter 7, a classification of irrationals based on various values for ε is described. The idea is to define a number x to be “more irrational” if the property still holds for larger values of ε. This leads to the introduction of a new family of irrationals: the transcendentals, studied in Chapters 7 and 8. Actually, if the property holds for ε>1, then x is a transcendental number. It’s been conjectured for a long time that π and e are transcendentals. However, the first number L to be proved to be transcendental was specially designed by Liouville to fit the results of Chapter 6. This construction is explained in Chapter 7: L is build such that, for any ε>0, there are infinitely many p and q such that |L-p/q|<1/q1+ε, and this proves that L is transcendental.

Finally, Chapter 9, 10 and 11 deal with more recent questions such as the problem of randomness in the decimal expansion of irrational numbers, and the first rigorous definitions of the set R of real numbers by Kossak, Cantor, Heine and Dedekind. Dedekind’s definition of a real number as a cut of the set of rationals became the classical one, but it is known that the other constructions are equivalent. The chapter about randomness is a bit short and unfortunately the recent approaches to define random sequences by Chaitin, Solovay and Martin-Löf are not mentioned. This part ends with some conclusion on the role of irrationals in modern mathematics.

This book contains a lot of fun for whoever likes mathematics. As it goes into details, I would recommend The irrationals: A story of the numbers you can’t count on particularly to students or to mathematicians non specialized in number theory, who would like to learn about its history – or just to enjoy some remarkably elegant proofs. From that perspective, some chapters like Chapters 6 and 10 are particularly successful.

As a side note, here is a terrific biography of Roger Apéry by his son, who is also a mathematician. When I was a student in Caen, Apéry was famous, both for his result and for having once forgotten his son (the same one?) on his motocycle in the parking lot of the university when supposedly driving him to school. (I [even more personally] find most interesting the description of the competition between the young ENS students, Roger Apéry and Jacqueline Lelong-Ferrand, for the first position at the agrégation final exam, since I had the privilege of having Madame Lelong-Ferrand as my professor of differential geometry in Paris 6, circa 1983…)

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