Counterexamples in Analysis (Dover Books on Mathematics)

It turns out that questions of the form "Does A always imply B?" entail proofs with two very different flavors, depending on whether the answer is affirmative or negative. The affirmative variety can be very difficult, as it usually deals with an infinity of things. But a negative answer requires only one solitary example of an A that is not a B; this is affectionately known as a "counter-example". These are the slickest little proofs around--often a one liner--and they can provide a lot of insight. Here's a trickier one: Are all linear functions continuous? Surprisingly, the answer is "no", which means there is a counter-example. Gelbaum and Olmsted show how to construct a discontinuous linear function. Case closed. They also provide examples of

A perfect nowhere dense set

A linear function space that is a lattice but not an algebra

A connected compact set that is not an arc

A divergent series whose general term approaches zero

A nonuniform limit of bounded functions that is not bounded

I won't give away any more (although there are hundreds). The book has chapters on real numbers, functions and limits, differentiation, sequences, infinite series, set and measure on the real axis, functions of two variables, metric and topological spaces, and more. Each section begins with a brief summary of the basic concepts and definitions, then launches into a list of terse counter-examples. This is simply indispensable for students of mathematical analysis, as it can help to explain why you cannot weaken those seemingly stringent hypotheses to various theorems; if you do, one of these quirky counter-examples will rush in and ruin your day. This is a great book to have on hand. I highly recommend it. (I won't tell you how it ends.)


Test the limits !   October 5, 2004
Palle E T Jorgensen (Iowa City, Iowa United States)
51 out of 53 found this review helpful

In my teaching of the basic tools of mathematical analysis; and even going back to my student days, I noticed the hurdle that separates the beautiful definitions from the `messy' examples. Often students tell me that the theory looks so easy, `but how do we construct an example to illustrate the limits of the theory?' -----A counter example?
Part of the difficulty is that the definitions involve quantifiers; and how do you check the quantifier `for all' ? And on top of that, there are the axioms of set theory: the axiom of choice, or one of its equivalent variants.

The lovely little book by Gelbaum-Olmsted was a savior to many of us when we started out in math, and it appeared first in 1961. But I had almost forgotten about it until by accident (while browsing in the bookstore) I stumbled over a new edition of it about a year ago, a lovely Dover reprinted edition. And so affordable !

In all the other books you learn about the wonderful things that are true about convergence, sets on the line or in the plane, modern variants of the so called Fundamental Theorem of Calculus, and in Gelbaum-Olmstead you learn the things that aren't true. And then there are all the lovely Cantor constructions, The Devil's Staircase, space filling curves, and much more; beautiful, but little known constructions going back to Lebesgue, and some to Riemann.

But more importantly the book gives students an edge when they have to do the assigned exercises in your analysis course. Many told me that the book is a 'secret weapon'.

Palle Jorgensen, October 2004.



A Bestiary of Analysis Monsters   December 9, 2003
43 out of 47 found this review helpful

For 200 years after it was invented by Isaac Newton, calculus lacked a rigorous foundation. In the 1800's the missing rigor was finally provided by the ingenious theory of limits, developed by Bolzano, Cauchy, Weierstrass, and others. This development, in turn, revealed the need to formulate and understand the structure of the real numbers. This structure was provided by Cantor, Dedekind, and Peano, who showed how the real numbers can be constructed from set theory.

But it was a Faustian bargain, because immediately a host of bizarre and counterintuitive examples were discovered - continuous functions that were nowhere differentiable, nonmeasurable sets, one-to-one pairing of points between the line and the plane. These peculiar entities were deeply disturbing to many.

Poincare said "Logic sometimes makes monsters. For half a century we have seen a mass of bizarre functions which appear to be forced to resemble as little as possible honest functions which serve some purpose... In former times when one invented a new function it was for a practical purpose; today one invents them purposely to show up defects in the reasoning of our fathers and one will deduce from them only that."

These counterexamples displayed features that were nowhere to be found in the physical universe. When Richard Feynman was a physics graduate student at Princeton, he would tease his mathematician friends that mathematics was so easy that he could instantly decide the truth or falsehood of any mathematical statement they could give him. One day they challenged him with the grand-daddy of all the paradoxes, the Banach-Tarski paradox: That the unit ball in R3 could be divided into a finite number of pieces, and the pieces could, by rigid translation and rotation, be reassembled into two unit balls. But they made a mistake: instead of saying "unit ball in R3", they said "apple". Feynman quickly pointed out that the nonmeasurable pieces, that they had so rigorously defined, must split apart even every electron of the apple.

When I was a graduate student in mathematics, "Counterexamples in Analysis" was my favorite book, and I had a lot of fun amazing my fellow graduate students by quoting from it. Since then, however, I have swung around more to the viewpoint of Poincare and Feynman: "Logic sometimes makes monsters." From either viewpoint, however, the counterexamples are immensely entertaining.


Great Book -- A "must have" for your bookshelf   August 20, 2004
ktrmes (New York, New York USA)
9 out of 10 found this review helpful

The counterexamples here are a wonderful aid to educating intuition about definitions in Real Variables. It may sound strange, but I always thought of this book as entertaining reading: If you glance at the table of contents, you'll may find youself saying, "wait, no, that can't -- well, I guess so, but what does that look like?" In later conversations you may find youself saying: "wait a second, I seem to recall seeing somewhere a continuous nowhere differentiable function," or someting of the sort. Unfortunately, there are not a whole lot of these creatures in the book, but they are worth spending some (enjoyable) time with.


Indispensable for students of real analysis   July 1, 2003
4 out of 8 found this review helpful

Graduate students of mathematics, just buy this book - no questions asked. You need the examples and this level of understanding. For the price there are just no questions - buy it!