For the honors student and aspiring mathematician, freshman year in college represents a right of passage, at least at the top-tier universities. He must set aside slovenly habits picked up in high school and learn to wield the predicate calculus in all its glory in order to construct rigorous proofs of non-trivial theorems. For many, this can be an excruciating sink-or-swim exercise. Analysis, on the foundation on which it was put by Cauchy and Weierstrass in the nineteenth century, involves arcane epsilontics that do not come naturally, although once acquired they constitute an effective tool in the analyst’s arsenal. For instance, a function f(x) is continuous at x if for all ε>0, there exists a δ>0 such that |f(x)-f(y)|<ε whenever |x-y|<δ. Statements of this kind involving concatenation of universal and existential quantifiers are common, and this one is among the simplest (merely to state the property of Cauchy completeness, for instance, involves a fearsome-looking proposition containing no less than eight interdependent quantifiers, five universal and three existential, plus one logical implication: ∀∃∀∀⇒∃∀∃∀). To the neophyte unaccustomed to such manipulations, an operation such as negating a statement containing more than one quantifier can be hard to perform, but practice is needed to become proficient at proving typical results (for instance, one will gather experience as to what direction to take in showing an implication, either directly or in the contrapositive). To acquire facility at juggling statements involving quantifiers and logical implication calls for a leap of the imagination, from monadic logic (Aristotle to Kant) to polyadic logic (post-Fregean).
At most schools, the sorting out of the wheat from the chaff takes place during an accelerated course in introductory analysis, which is well suited to the purpose (in recent years, at some places a preparatory course on techniques of proof such as induction has come to be inserted beforehand, but in this reviewer’s opinion that is just a waste of good students’ time). This reviewer’s textbook for the spring semester of his freshman year was Walter Rudin’s legendary Principles of Mathematical Analysis. Affectionately known as ‘baby Rudin’ in order to distinguish it from the graduate-level textbook on real and complex analysis by the same author (‘papa Rudin’), this text has been a staple in the curriculum for generations and many will swear by it. It takes its origin from when, as a graduate student back in the 1950’s, Rudin had to teach analysis to advanced undergraduates and, finding there to be no satisfactory text, decided to write his own. Over the course of his career he tinkered with and perfected it, and it has achieved classic status in its third and final edition (in the McGraw-Hill international series in pure and applied mathematics, the entries of which are outrageously expensive, by the way).
Sketch of contents: the first chapter is given over to Dedekind’s arithmetical construction of the real number line from the rationals and the least-upper-bound property (it can easily be skipped). The course gets underway in earnest in chapter two with basic topology in metric spaces and properties of denumerable sets. Chapter three covers numerical sequences and series (convergence, root and ratio tests, rearrangements). These simple notions, later to be generalized to other spaces than just the real number line, are basic to all of analysis. At last in chapter four we arrive at the epsilon-delta definition of limits, continuity, compactness, uniform continuity, connectedness and infinity. Here is where the going gets hard for the inexperienced student. Chapters five and six introduce the derivative and the Riemann-Stieltjes integral. The material will be for the most part familiar to those who have had calculus at the high-school level (at least formally), but the proofs entirely novel. The same cannot be said of the next two chapters, seven and eight, on sequences and series of functions. The old and trusted concepts are now applied to function spaces equipped with a metric. Any serious mathematician will have to master this abstract point of view sooner or later (it dates to the early decades of the twentieth century, a revolutionary period when the foundations of topology and functional analysis in their modern form were laid). As an index of the complications one encounters, one meets the need for the concepts of uniform convergence and how it relates to continuity, differentiation, equicontinuous families and compactness. Chapter seven culminates in a nice and thorough proof of the Stone-Weierstrass theorem on approximation of continuous functions by polynomials, a foundational result of analysis. Chapter eight covers special functions such as the exponential, logarithm, gamma function and elementary Fourier series.
Everyone is agreed that the first eight chapters in Rudin’s textbook constitute almost the canonical exposition of introductory analysis. There seems to be a consensus that the remaining three chapters on multivariate functions, integration of differential forms and Stokes’ theorem and beginning measure theory up to the Lebesgue integral are less strong. Certainly, there are other good places to get acquainted with these subjects, but in this reviewer’s opinion, however, there is nothing wrong with Rudin’s treatment, although to be sure it looks more like an analyst’s view than a geometer’s. The big theorems receive efficient proofs: implicit function theorem, inverse function theorem, rank theorem, Stokes’ theorem. As to the latter, it should be remarked that Rudin employs an idiosyncratic definition of differential forms that is just sufficient to enable him to obtain their elementary properties and derive Stokes’ theorem, but which completely elides their intuitive geometrical significance, which one would learn in a standard course on differentiable manifolds (no manifolds here, everything is done in an open subset of Euclidean space). Suffice it to say that these three chapters are too concise and by no means adequate to their respective subjects. This observation is particularly true of the last chapter, which breezes through measure theory and the Lebesgue integral up to the Riesz-Fischer theorem in just thirty pages! Half of an ordinary textbook on real analysis could be devoted to these subjects alone. If one is willing to bear with Rudin and view these chapters on their own terms as a thought-experiment, possibly their value, which consists in seeing how Rudin does things, could be appreciated. The problems in these chapters are just as good as in the earlier ones.
What sets ‘baby Rudin’ apart from the crowd is its excellent style. Far more than the physicist, the mathematician is apt to set to work as a craftsman and to hold himself responsible to produce a work of art, and Rudin is an acknowledged master. In a scientific field in which knowledge is to be conveyed, the medium cannot be the entire message, but it is certainly a part of it. The hallmarks of Rudin’s style are concision and elegance; he will not content himself with proving a result in any which way, but strives for a pleasing economy. In contrast, Pugh and to a lesser extent Apostol and Abbott in their textbooks on introductory analysis seem to take the view that the more said, the more help it will be to the student (especially the beginner or less proficient). Now, in any technical exposition, there will always be considerable leeway for a judgment call about what to leave tacit; authors differ, but when everything is thrown in in the hope that enough if it will stick in the reader’s mind and facilitate his comprehension, the effect can be rather tedious (Arnold, on the other hand, is an example of someone who says too little for ordinary mortals, leaving them with the illusion of understanding without adequate erudition). Rudin takes care to put in enough, but not too much. The discerning reader should be on the lookout for subtleties and when one is encountered, trust that he can resolve it to his own satisfaction with a moment’s thought. The budding mathematician must ever cultivate his sense of mathematical maturity and here is a suitable place to begin, at the level of introductory analysis. Such attention to detail is really part of the art of any communication, as Fowler tirelessly seeks to get across throughout his Modern English Usage.
The homework exercises—some 289 of them over eleven chapters—are legendary for their difficulty, but this may be mainly a function of the fact that callow students are surprised when called upon to prove something for real for the first time. In retrospect to someone who has gained exposure to the subject at the graduate level, they appear often rather challenging but not quite impossible if one be willing to make a determined effort. This reviewer remembers, for instance, wrapping his mind for hours around the problem to find a non-empty perfect set on the real line that contains no rational number. Rudin’s problems tend to be conceptual rather than computational; they are selected to illustrate a point and supporting calculations are secondary in importance and only very seldom do they become very involved. In contrast, many problems in Courant and John seem to have little justification other than to prove that one can see a demanding and byzantine calculation through to its successful conclusion, as if mastering mathematics were a signaling game. For a nice specimen of Rudin’s preferences, see Problem 7.14 in which one is asked to show that a given continuous map takes the real line onto the unit square (in fact, it maps the Cantor set onto the unit square).
In sum: want to become a real mathematician? Hit the books hard with the classic baby Rudin or condemn yourself forever to mediocrity!