# BIG Math Network Blog:

I hope to start writing this blog regularly, if not frequently, but, firstly, I hope the following blog helps many of my readers too:

https://bigmathnetwork.wordpress.com/

BIG Math Network: Connecting Mathematical Scientists in Business, Industry, Government and Academia

(I just came across today from an AMS Blog.)

(Thanks to Evelyn J. Lamb)

— shared here by Nalin Pithwa.

# The Benefits of Tutoring: More Than Extra Income

How tutoring gives satisfaction and meaning during grad school.

# Misteaks of mathematicians

Misteaks.

(As remembered by Prof Walter Rudin in his autobiography, “The Way I Remember it”.)

Mathematicians are human. Humans make mistakes. Therefore…

This is no cause for alarm. I have no figures to back this up, but compared to the flood of published papers, the number of serious errors in the literature must be tiny. Most are probably caught by referees. And, if there is a serious error in a paper that is important enough to be studied by a significant number of interested mathematicians, that error will be discovered. Even better, the one who made it won’t be able to argue his way out of it.

There is an amusing article by Geoffrey K. Pullum in Natural Language and Linguistic Theory 5, 1987, 303-309, which compares this social aspect of mathematics with what happens in linguistics. It describes the story of Rourke’s claim to have proved the Poincare conjecture. (The article was sent to me by Catherine, my linguist daughter.)

My first encounter with this sort of thing started with the letter on the following page (typed a bit later after two paragraphs):

Needless to say, I was totally amazed. Here was Dieudonne, a world class mathematician and one of the founders of Bourbaki, not telling me, a young upstart, “you are wrong, because here is what I proved a few years ago” but asking me, instead to tell him what he had done wrong! Actually, it took me a while to find the error, and if I had not proved earlier (in J. Math. Mech. 7, 1958, 103-116) that convolution-factorization is always possible in $L^{1}$ I would have accepted his conclusion with no hesitation, not because he was famous, but because his argument was simple, and, perfectly correct, as far as it went.

He proved, correctly, that every convolution of non-negative functions coincides almost everywhere with one that is lower semicontinuous. But (and this is what he ignored) that function may have $+\infty$ among its values.

NORTHWESTERN UNIVERSITY

Evanston, Illinois.

58, Rue de Verneuil, Paris 7e (France).

Paris, December 17

Dear Professor Rudin:

In the last issue of the Bulletin AMS, I see that you announce in abstract 7311, p.382, that in the algebra $L^{1}(R^{n})$, any element is the convolution of two elements of that algebra. I am rather amazed at the statement, for a few years ago I had made a simple remark which seemed to me to disprove your theorem (Compositio Mathematica, 12 (1954) p.17, footnote 3). I reproduce the proof for your convenience.

Suppose f, g are in $L^{1}$ and greater than or equal to zero, and for each n consider the usual “truncated” functions $f_{n}=\inf(f,n)$ and $g_{n}=\inf(g,n)$; f(resp. g) is the limit of the increasing sequence $(f_{n})$ (resp. g_{n}), hence, by the usual Lebesgue convergence theorem, $h=f*g$ is a.e. the limit of

$h_{n}=f_{n}*g_{n}$, which is obviously an increasing sequence. Moreover, $f_{n}$ and $g_{n}$ are both in $L^{2}$, hence it is well known that $h_{n}$ can be taken continuous and bounded. It follows that h is a.e. equal to a Baire function of the first class. However, it is well-known that there are integrable functions which do not have that property, and therefore they cannot be convolutions.

I am unable to find any flaw in that argument, and if you can do so, I would very much appreciate if you can tell me where I am wrong.

Sincerely yours

J. Dieudonne

His argument proves that there are non-negative functions h in

$L^{1}$ that are not representable as $h=f*g$ with $f \geq 0$ and $g \geq 0$. (I had also observed this). In the general real-valued case, if $h=f*g$, each of f and g is a difference of two non negative functions, so that $f*g$ breaks into four convolutions of the type considered by Dieudonne. Of these, two are greater than or equal to zero, two are less than or equal to zero, and one may therefore run into the problem of subtracting $\infty$ from $\infty$ (which is at least as much as of a no-no as is dividing 0 by 0). Hence, one can no longer conclude that h coincides almost everywhere with a function of Baire class one, that is, with a real-valued function which is everywhere the pointwise limit of a sequence of continuous ones.

Dieudonne had fallen into the “without loss of generality” trap by restricting himself to $f \geq 0$ and $g \geq 0$, and tacitly assuming that the general case would follow.

Here is his  reply to my explanation.

Paris, January 12, 1952.

Dear Professor Rudin:

Thank you for pointing out my error; as, it is of a very common type, I suppose I should have been able to detect it myself, but you know how hard it is to see one’s own mistakes, when you have once become convinced that some result must be true!!

Your proof is very ingenious; I hope you will be able to generalize that result to arbitrary locally compact abelian groups, but I suppose this would require a somewhat different type of proof.

With my congratulations for your nice result and my best thanks, I am

Sincerely yours

J. Dieudonne

The factorization theorem was indeed extended, even further than he had hoped. When Paul Cohen saw my rather complicated proof about $L^{1}(\Re^{n})$ (the case n=1 was much easier) he said: “Aha, approximate identities” and quickly produced a very general factorization theorem in Banach algebras with approximate left identities (Duke Math. J. 26, 1959, 199-205). Ed Hewitt (Math. Scand. 15, 1964, 147-155) extended Cohen’s proof so as to include convolution operators on $L^{p}$ ($1 \leq p < \infty$). Every h in $L^{p}$ is $f*g$ with f in $L^{1}$, g in $L^{p}$.

In my next story, I was the one who goofed, but it ended well. This concerned the open unit ball B in the n-dimensional complex space $C^{n}$. A one-to-one holomorphic map from B onto B will be called an automorphism of B. (When $n=1$, B is the unit disc in C, and its automorphisms are the familiar Moebius transformations that send z to $e^{i\theta}\frac{(z-\alpha)}{(1-\overline{\alpha} z)}$ ). The automorphisms of B are also explicitly known for all n.

Can a space X of complex valued functions on B(or on the boundary of B)  Moebius-invariant (or $\mathcal{M}$ invariant) spaces of certain types (Duke Math. J. 43, 10976, 841-861) but the following was not answered:

Which closed subalgebras of C(B) are $\mathcal{M}- invariant$?

Here, C(B) is the algebra of all complex valued continuous (possibly unbounded) functions on B, with the topology of uniform convergence on compact sets, and with pointwise addition and multiplication.

The five obvious possibilities are: $\{ 0 \}$, the constants, the holomorphic functions on B, those whose complex conjugates are holomorphic and C(B) itself. The answer (Ann. Inst. Fourier 23, 1983, 19-41) is:

Theorem. There are no others.

I believe that this is the most difficult theorem that I ever proved. It was new even for $n=1$. I started with a proof of the one-dimensional case, and then used that to derive the same conclusion in n dimensions. Fairly soon after I submitted the paper, Malgrange, who was an editor of Annales Fourier, wrote that the referee did not understand how I passed from 1 to n. When I looked at it, I couldn’t understand it either! What I had written simply made no sense, and there seemed to be no way to repair it. I had to go back and instead of first dealing with the one-variable case I had to do the whole thing in n variables from the start. Fortunately, it worked. But it took a whole summer.

When I sent the corrected (much longer) version to Malgrange, I wrote that I should like to  thank the referee in the corrected paper, but only if I could mention his or her name. I saw no virtue in anonymous thanks. The referee agreed to this; it turned out to be my friend Jean-Pierre Rosay.

A few years later, we (i.e., the Madison Mathematics Department) wanted to invite Rosay for a whole year’s visit. I had a so-called Vilas Professorship which provided research funds for worthy projects. So I sent a request to the appropriate committee; to strengthen the case, I mentioned that not only I but several of my colleagues (Ahern, Forelli, Nagel, Wainger among them) would find him very stimulating. My letter was returned by no other than Irv Shain, the Chancellor, saying that Vilas Professorships were only for the benefit of those who  had one and for no one else’s, and that I should write a different letter, explaining how Rosay’s presence would benefit me. I did that, and as a clincher enclosed a reprint of the paper in which I thanked him. That did it.

Soon after he arrived, he and I started  to work on several questions about holomorphic maps. This resulted in a long paper. (Trans. AMS 310, 1988, 47-86), the first of several that we wrote together. Our collaboration went so well that I suggested that we ought to try to  keep him. We succeeded in this, and it could well be that my role in getting him to stay here was one of thei best things I ever did for the Department.

I know of only one totally absurd paper that was published in a respectable journal, namely the one by Nikola Pandeski in Math., Annalen 287, 1990, 185-192. In one variable, the “corona theorem” asserts that the open unit disc U is dense in the maximal ideal space of the Banach algebra of all bounded holomorphic functions in U. Its original proof, by Lennart Carleson (Annals of Math. 76, 1963, 547-559) involved a great deal of difficult “hard” analysis. A much simpler one was later found by Tom Wolff. But the n-variable analogue, which Pandeski claimed to have proved, is still wide open.

This paper appeared during a several complex variables conference in Oberwolfach. I heard that it caused great hilarity because nothing in it makes sense. For example, the proof starts by covering a sphere with a finite disjoint collection of small balls! I have heard several explanations about how this absurdity got into print, none of them convincing. I was quite annoyed about the whole affair because Pandeski attributed several absolutely false assertions to me, and because Granert, the editor who  had (mis)handled this paper refused to publish my protest.

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Never mind. To err is human …

Nalin Pithwa

# Mathematics versus other escapes from reality

Of all escapes from reality, Mathematics is the most successful ever. It is a fantasy that becomes all the more addictive because it works back to improve the same reality we are trying to evade. All other escapes — sex, drugs, hobbies, whatever — are ephemeral by comparison. The mathematician’s feeling of triumph, as he/she forces the world to obey the laws his/her imagination has created feeds on its own success. The world is permanently changed by the workings of his/her mind, and the certainty that his/her creations will endure renews his/her confidence as no other pursuit.

Gian-Carlo Rota, an MIT Mathematician.

# Function Algebras — by Walter Rudin

(The following is reproduced from the book “The Way I Remember It” by Walter Rudin. The purpose is just to share the insights of a formidable analyst with the student community.)

When I arrived at MIT in 1950, Banach algebras were one of the hot toppers. Gelfand’s 1941 paper “Normierte Ringe” had apparently only reached the USA in the late forties, and circulated on hard-to-read smudged purple ditto copies. As one application of the general theory presented there, it contained a stunningly short proof of Wiener’s lemma: the Fourier series of the reciprocal of a nowhere vanishing function with absolutely convergent Fourier series also  converges absolutely. Not only was the proof extremely short, it was one of those that are hard to forget. All one needs to remember is that the absolutely convergent Fourier series form a Banach algebra, and that every multiplicative linear functional on this algebra is evaluation at some point of the unit circle.

This may have led some to believe that Banach algebras would now solve all our problems. Of course, they could not, but they did provide the right framework for many questions in analysis (as does most of functional analysis) and conversely, abstract questions about Banach algebras often gave rise to interesting problems in “hard analysis”. (Hard analysis is used here as Hardy and Littlewood used it. For example, you do hard analysis when, in order to estimate some integral, you break it into three pieces and apply different inequalities to each.)

One type of Banach algebras that was soon studied in detail were the so-called function algebras, also known as uniform algebras.

To see what these are, let $C(X)$ be the set of all complex-valued continuous functions on a compact Hausdorff space X. A function algebra on X is a subset A of $C(X)$ such that

(i) If f and g are in A, so are $f+g$, $fg$, and $cf$ for every complex number c (this says that A is an algebra).

(ii) A contains the constant functions.

(iii) A separates points on X (that is, if $p \neq q$, both in X, then $f(p) \neq f(q)$ for some f in A), and

(iv) A is closed, relative to the sup-norm topology of $C(X)$, that is, the topology in which convergence means uniform convergence.

A is said to be self-adjoint if the complex conjugate of every f in A is also in A. The most familiar example of a non-self-adjoint function algebra is the disc algebra $A(U)$ which consists of all f in $C(U)$ that are holomorphic in U. (here, and later, U is the open unit disc in C, the complex plane, and $\overline{U}$ is its closure). I already had an encounter with  $A(U)$, a propos maximum modulus algebras.

One type of question that was asked over and over again was: Suppose that a function algebra on X satisfies … and …and is it C(X)? (In fact, 20 years later a whole book, entitled “Characterizations of C(X) among its Subalgebras”  was published by R. B. Burckel.)  The Stone-Weierstrass Theorem gives the classical answer. Yes, if A is self-adjoint.

There are problems even when X is a compact interval I on the real line. For instance, suppose A is a function algebra on I, and to every maximal ideal M of A corresponds a point p in I such that M is the set of all f in A having $f(p)=0$ (In other words, the only maximal ideals of A are the obvious ones). Is $A=C(I)$? This is still unknown, in 1995.

If $f_{1}, f_{2}, \ldots f_{n}$ are in $C(I)$, and the n-tuple $(f_{1}, f_{2}, \ldots, f_{n})$ separates points on I, let $A(f_{1}, \ldots , f_{n})$ be the smallest closed subalgebras of $C(I)$ that contains $f_{1}, f_{2}, \ldots f_{n}$ and I.

When $f_{1}$ is 1-1 on I, it follows from an old theorem of Walsh (Math. Annalen 96, 1926, 437-450) that $A(f_{1})=C(f)$.

Stone-Weierstrass implies that $A(f_{1}, f_{2}, \ldots, f_{n})=C(I)$ if each $f_{i}$ is real-valued.

In the other direction, John Wermer showed in Annals of Math. 62, 1955, 267-270, that $A(f_{1}, f_{2}, f_{3})$ can be a proper subset of $C(I)$!

Here is how he did this:

Let E be an arc in C, of positive two-dimensional measure, and let $A_{E}$ be an algebra of all continuous functions on the Riemann sphere S (the one-point compactification of C). which are holomorphic in the complement of E. He showed that $g(E)=g(S)$ for every g in $A_{E}$, that $A_{E}$ contains a triple $(g_{1}, g_{2}, g_{3})$ that separates points on S and that the restriction of $A_{E}$ to E is closed in $C(E)$. Pick a homeomorphism $\phi$ of I onto E and define $f_{i}=g_{i} \circ \phi$. Then, $A(f_{1}, f_{2}, f_{3}) \neq C(I)$, for if h is in $A(f_{1}, f_{2}, f_{3})$ then $h= g \circ \phi$ for some g in $A_{E}$, so that

$h(I)=g(E)=g(S)$ is the closure of an open subset of $C$ (except when h is constant).

In order to prove the same with two function instead of three I replaced John’s arc E with a Cantor set K, also of positive two-dimensional measure (I use the term “Cantor set” for any totally disconnected compact metric space with no isolated points; these are all homeomorphic to each other.) A small extra twist, applied to John’s argument, with $A_{K}$ in place of $A_{E}$, proved that $A(f_{1}, f_{2})$ can also be smaller than $C(I)$.

I also used $A_{K}$ to show that $C(K)$ contains maximal closed point-separating subalgebras that are not maximal ideals, and that the same is true for $C(X)$ whenever X contains a Cantor set. These ideas were pushed further by Hoffman and Singer in Acta Math. 103, 1960, 217-241.

In the same paper, I showed that $A(f_{1}, f_{2}, \ldots, f_{n})=C(I)$ when $n-1$ of the n given functions are real-valued.

Since Wermer’s paper was being published in the Annals, and mine strengthened his theorem and contained other interesting (at least to me) results, I sent mine there too. It was rejected, almost by return mail, by an anonymous editor, for not being sufficiently interesting. I have had a few others papers rejected over the years, but for better reasons. This one was published in Proc. AMS 7, 1956, 825-830, and is one of six whose Russian transactions were made into a book “Some Questions in Approximation Theory”, the others were three by Bishop and two by Wermer. Good company.

Later, Gabriel Stolzenberg (Acta Math. 115, 1966, 185-198) and Herbert Alexander (Amer. J. Math., 93, 1971, 65-74) went much more deeply into these problems. One of the highlights in Alexander’s paper is:

$A(f_{1}, f_{2}, \ldots f_{n})=C(I)$ if $f_{1}, f_{2}, \ldots, f_{n-1}$ are of bounded variation.

A propos the Annals (published by Princeton University) here is a little Princeton anecdote. During a week that I spent there, in the mid-eighties, the Institute threw a cocktail party. (What I enjoyed best at that affair was being attacked by Armand Borel for having said, in print, that sheaves had vanished into the background.) Next morning I overheard the following conversation in Fine Hall:

Prof. A: That was a nice party yesterday, wasn’t it?

Prof. B: Yes, and wasn’t it nice that they invited the whole department.

Prof. A: Well, only the full professors.

Prof. B: Of course.

The above-mentioned facts about Cantor sets led me to look at the opposite extreme, the so-called scattered spaces. A compact Hausdorff space Q is said to be shattered if Q contains no perfect set, every non-empty compact set F in Q thus contains a point that is not a limit point of F. The principal result proved in Proc. AMS 8, 1957, 39-42 is:

THEOREM: Every closed subalgebra of $C(Q)$ is self-adjoint.

In fact, the scattered spaces are the only ones for which this is true, but I did not state this in that paper.

In 1956, I found a very explicit description of all closed ideals in the disc algebra $A(U)$ (defined at the beginning of this chapter). The description involves inner function. These are the bounded holomorphic functions in U whose radial limits have absolute value 1 at almost every point of the unit circle $\mathcal{T}$. They play a very important role in the study of holomorphic functions in U (see, for instance, Garnett’s book, Bounded Analytic Functions) and their analogues will be mentioned again, on Riemann surfaces, in polydiscs, and in balls in $C^{n}$.

Recall that a point $\zeta$ on $\mathcal{T}$ is called a singular point of a holomorphic  function f in U if f has no analytic continuation to any neighbourhood of $\zeta$. The ideals in question are described in the following:

THEOREM: Let E be a compact subset of $\mathcal{T}$, of Lebesgue measure 0, let u be an inner function all of whose singular points lie in E, and let $J(E,u)$ be the set of all f in $A(U)$ such that

(i) the quotient f/u is bounded in U, and

(ii) $f(\zeta)=0$ at every $\zeta$ in E.

Then, $J(E,u)$ is a closed ideal of A(U), and every closed ideal of $A(U) (\neq \{0\})$ is obtained in this way.

One of several corollaries is that every closed ideal of A(U) is principal, that is, is generated by a single function.

I presented this at the December 1956 AMS meeting in Rochester, and was immediately told by several people that Beurling had proved the same thing, in a course he had given at Harvard, but had not published it. I was also told that Beurling might be quite upset at this, and having Beurling upset at you was not a good thing. Having used this famous paper about the shift operator on a Hilbert space as my guide, I was not surprised that he too had proved this, but I saw no reason to withdraw my already submitted paper. It appeared in Canadian J. Math. 9, 1967, 426-434. The result is now known as Beurling-Rudin theorem. I met him several times later, and he never made a fuss over this.

In the preceding year Lennart Carleson and I, neither of us knowing what the other was doing proved what is now known as Rudin-Carleson interpolation theorem. His paper is in Math. Z. 66, 1957, 447-451, mine in Proc. AMS 7, 1956, 808-811.

THEOREM. If E is a compact subset of $\mathcal{T}$, of Lebesgue measure 0, then every f in C(E) extends to a function F in A(U).

(It is easy to see that this fails if $m(E)>0$. To say that F is an extension of f means simply that $F(\zeta)=f(\zeta)$ at every $\zeta$ in E.)

Our proofs have some ingredients in common, but they are different, and we each proved more than is stated above. Surprisingly, Carleson, the master of classical hard analysis, used a soft approach, namely duality in Banach spaces, and concluded that F could be so chosen that $||F||_{U} \leq 2||f||_{E}$. (The norms are sup-norms over the sets appearing as subscripts.) In the same paper he used his Banach space argument to prove another interpolation theorem, involving Fourier-Stieltjes transforms.

On the other hand, I did not have functional analysis in mind at all, I did not think of the norms or of Banach spaces, I proved, by a bare-hands construction combined with the Riemann mapping theorem that if $\Omega$ is a closed Jordan domain containing $f(E)$ then f can be chosen so that $F(\overline{U})$ also lies in $\Omega$. If $\Omega$ is a disc, centered at 0, this gives $||F||_{U}=||f||_{E}$, so F is a norm-preserving extension.

What our proofs had in common is that we both used part of the construction that was used in the original proof of the F. and M. Riesz theorem (which says that if a measure $\mu$ on $\mathcal{T}$ gives $\int fd\mu=0$ for every f in $A(U)$ then $\mu$ is absolutely continuous with respect to Lebesgue measure). Carleson showed, in fact, that F. and M. Riesz can be derived quite easily from the interpolation theorem. I tried to prove the implication in the other direction. But that had to wait for Errett Bishop. In Proc. AMS 13, 1962, 140-143, he established this implication in a very general setting which had nothing to do with holomorphic functions or even with algebras, and which, combined with a refinement due to Glicksberg (Trans. AMS 105, 1962, 415-435) makes the interpolation theorem even more precise:

THEOREM: One can choose F in $A(U)$ so that $F(\zeta)=f(\zeta)$ at every $\zeta$ in E, and $|f(z)|<||f||_{E}$ at every z in $\overline{U}\\E$.

This is usually called peak-interpolation.

Several variable analogues of this and related results may be found in Chap. 6 of my Function Theory in Polydiscs and in Chap 10 of my Function Theory in the Unit Ball of $C^{n}$.

The last item in this chapter concerns Riemann surfaces. Some definitions are needed.

A finite Riemann surface is a connected open proper subset R of some compact Riemann surface X, such that the boundary $\partial R$ of R in X is also the boundary of its closure $\overline{R}$ and is the union of finitely many disjoint simple closed analytic curves $\Gamma_{1}, \ldots, \Gamma_{k}$. Shrinking each $\Gamma_{i}$ to a point gives a compact orientable manifold whose genus g is defined to be the genus of R. The numbers g and k determine the topology of R, but not, of course, its conformal structure.

$A(R)$ denotes the algebra of all continuous functions on $\overline{R}$ that are holomorphic in R. If f is in $A(R)$ and $|f(p)|=1$ at every point p in $\partial R$ then, just as in U, f is called inner. A set $S \subset A(R)$ is unramified if every point of $\overline{R}$ has a neighbourhood in which at least one member of S is one-to-one.

I became interested in these algebras when Lee Stout (Math. Z., 92, 1966, 366-379; also 95, 1967, 403-404) showed that every $A(R)$ contains an unramified triple of inner functions that separates points on $\overline{R}$.  He deduced from the resulting embedding of R in $U^{S}$ that $A(R)$ is generated by these 3 functions. Whether every $A(R)$ is generated by some pair of its member is still unknown, but the main result of my paper in Trans. AMS 150, 1969, 423-434 shows that pairs of inner functions won’t always do:

THEOREM: If $A(\overline{R})$ contains a point-separating unramified pair f, g of inner functions, then there exist relatively prime integers s and t such that f is s-to-1 and g is t-to-1 on every $\Gamma_{i}$, and

(*) $(ks-1)(kt-1)=2g+k-1$

For example, when $g=2$ and $k=4$, then (*) holds for no  integers s and t. When $g=23$ and $k=4$, then $s=t=2$ is the only pair that satisfies (*) but it is not relatively prime. Even when $R=U$ the theorem gives some information. In that case, $g=0, k=1$, so (*) becomes $(s-1)(t-1)=0$, which means:

If a pair of finite Blaschke products separates points on $\overline{U}$ and their derivatives have no  common zero in U, then at least one of them is one-to-one (that is, a Mobius transformation).

There are two cases in which (*) is not  only necessary but also sufficient. This happens when $g=0$ and when $g=k=1$.

But there are examples in which the topological condition (*) is satisfied even though the conformal structure of R prevents the existence of a separating unramified pair of inner functions.

This paper is quite different from anything else that I have ever done. As far as I know, no  one has ever referred to it, but I had fun working on it.

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More blogs from Rudin’s autobiography later, till then,

Nalin Pithwa

# Interchanging Limit Processes — by Walter Rudin

As I mentioned earlier, my thesis (Trans. AMS 68, 1950, 278-363) deals with uniqueness questions for series of spherical harmonics, also known as Laplace series. In the more familiar setting of trigonometric series, the first theorem of the kind that I was looking for was proved by Georg Cantor in 1870, based on earlier work of Riemann (1854, published in 1867). Using the notations

$A_{n}(x)=a_{n} \cos{nx}+b_{n}\sin{nx}$,

$s_{p}(x)=A_{0}+A_{1}(x)+ \ldots + A_{p}(x)$, where $a_{n}$ and $b_{n}$ are real numbers. Cantor’s theorem says:

$If \lim_{p \rightarrow \infty}s_{p}(x)=0$ at every real x, then $a_{n}=b_{n}=0$ for every n.

Therefore, two distinct trigonometric series cannot converge to the same sum. This is what is meant by uniqueness.

My aim was to prove this for spherical harmonics and (as had been done for trigonometric series) to whittle away at the hypothesis. Instead of assuming convergence at every point of the sphere, what sort of summability will do? Does one really need convergence (or summability) at every point? If not, what sort of sets can be omitted? Must anything else be assumed at these omitted points? What sort of side conditions, if any, are relevant?

I came up with reasonable answers to these questions, but basically the whole point seemed to be the justification of the interchange of some limit processes. This left me with an uneasy feeling that there ought to be more to Analysis than that. I wanted to do something with more “structure”. I could not  have explained just what I meant by this, but I found it later when I became aware of the close relation between Fourier analysis and group theory, and also in an occasional encounter with number theory and with geometric aspects of several complex variables.

Why was it all an exercise in interchange of limits? Because the “obvious” proof of Cantor’s theorem goes like this: for $p > n$,

$\pi a_{n}= \int_{-\pi}^{\pi}s_{p}(x)\cos{nx}dx = \lim_{p \rightarrow \infty}\int_{-\pi}^{\pi}s_{p}(x)\cos {nx}dx$, which in turn, equals

$\int_{-\pi}^{\pi}(\lim_{p \rightarrow \infty}s_{p}(x))\cos{nx}dx= \int_{-\pi}^{\pi}0 \cos{nx}dx=0$ and similarly, for $b_{n}$. Note that $\lim \int = \int \lim$ was used.

In Riemann’s above mentioned paper, the derives the conclusion of Cantor’s theorem under an additional hypothesis, namely, $a_{n} \rightarrow 0$ and $b_{n} \rightarrow 0$ as $n \rightarrow \infty$. He associates to $\sum {A_{n}(x)}$ the twice integrated series

$F(x)=-\sum_{1}^{\infty}n^{-2}A_{n}(x)$

and then finds it necessary to prove, in some detail, that this series converges and that its sum F is continuous! (Weierstrass had not yet invented uniform convergence.) This is astonishingly different from most of his other publications, such as his paper  on hypergeometric functions in which mind-boggling relations and transformations are merely stated, with only a few hints, or  his  painfully brief paper on the zeta-function.

In Crelle’s J. 73, 1870, 130-138, Cantor showed that Riemann’s additional hypothesis was redundant, by proving that

(*) $\lim_{n \rightarrow \infty}A_{n}(x)=0$ for all x implies $\lim_{n \rightarrow \infty}a_{n}= \lim_{n \rightarrow \infty}b_{n}=0$.

He included the statement: This cannot be proved, as is commonly believed, by term-by-term integration.

Apparently, it took a while before this was generally understood. Ten years later, in Math. America 16, 1880, 113-114, he patiently explains the differenence between pointwise convergence and uniform convergence, in order to refute a “simpler proof” published by Appell. But then, referring to his second (still quite complicated) proof, the one in Math. Annalen 4, 1871, 139-143, he sticks his neck out and writes: ” In my opinion, no further simplification can be achieved, given the nature of ths subject.”

That was a bit reckless. 25 years later, Lebesgue’s dominated convergence theorem became part of every analyst’s tool chest, and since then (*) can be proved in a few lines:

Rewrite $A_{n}(x)$ in the form $A_{n}(x)=c_{n}\cos {(nx+\alpha_{n})}$, where $c_{n}^{2}=a_{n}^{2}+b_{n}^{2}$. Put

$\gamma_{n}==\min \{1, |c_{n}| \}$, $B_{n}(x)=\gamma_{n}\cos{(nx+\alpha_{n})}$.

Then, $B_{n}^{2}(x) \leq 1$, $B_{n}^{2}(x) \rightarrow 0$ at every x, so that the D. C.Th., combined with

$\int_{-\pi}^{\pi}B_{n}^{2}(x)dx=\pi \gamma_{n}^{2}$ shows that $\gamma_{n} \rightarrow 0$. Therefore, $|c_{n}|=\gamma_{n}$ for all large n, and $c_{n} \rightarrow 0$. Done.

The point of all this is that my attitude was probably wrong. Interchanging limit processes occupied some of the best mathematicians for most of the 19th century. Thomas Hawkins’ book “Lebesgue’s Theory” gives an excellent description of the difficulties that they had to overcome. Perhaps, we should not be too surprised that even a hundred years later many students are baffled by uniform convergence, uniform continuity etc., and that some never get it at all.

In Trans. AMS 70, 1961,  387-403, I applied the techniques of my thesis to another problem of this type, with Hermite functions in place of spherical harmonics.

(Note: The above article has been picked from Walter Rudin’t book, “The Way I  Remember It)) — hope it helps advanced graduates in Analysis.

More later,

Nalin Pithwa

# Pure versus Applied Mathematics

Relations between pure and applied mathematicians are based on trust and understanding. Pure mathematicians do not trust applied mathematicians, and applied mathematicians do not understand pure mathematicians. — Ian Stewart