## miércoles, 21 de junio de 2017

### Edmund Landau, the irrepressible spoiled child of mathematics

... Landau was an instance of that uncommon phenomenon, the scion of a wealthy family who yet had a powerful work ethic and a record of great achievement in a non-commercial field. Landau's mother Johanna, née Jacoby, came from a rich banking family. His father was a Professor of Gynecology in Berlin, with a successful practice. Landau Senior was also a keen supporter of Jewish causes. The family home was at Pariser Platz 6a, in the most elegant quarter of Berlin, close to the Branderburg Gate. Edmund was appointed to a professorship at Göttingen in 1909. When people asked for directions to his house, he would reply "You can't miss it. It's the finest house in town." He followed his father's (and Hadamard's) interest in Zionism, helping to establish the Hebrew University of Jerusalem and giving the first math lecture there, in Hebrew, shortly after the university opened in April 1925.

Landau was something of a character--this was a great age for mathematical characters--and there are apocrypha about him rivaling those of Hilbert and Hardy. Perhaps the best-known story is his remark about Emmy Noether, a colleague at Göttingen. Noether was mannish and very plain. Asked if she was not an instance of a great female mathematician, Landau replied: "I can testify that Emmy is a great mathematician, but that she is a female, I cannot swear." His work ethic was legendary. It is said that when one of his junior lecturers was in hospital, recuperating from a serious illness, Landau climbed a ladder and pushed a huge folder of work through the poor man's window. According to Littlewood, Landau simply did not know what it was like to be tired...

References
J. Derbyshire, Prime obsession: B. Riemann and the greatest unsolved problem in mathematics. Published by Plume (a member of Penguin Group), USA, 2003, pp. 230-231.

## lunes, 5 de junio de 2017

### Revisitando la demostración de Erdös del postulado de Bertrand

Al releer recientemente el artículo donde Erdös expone su prueba del postulado de Bertrand, caí en la cuenta de que el buen Paul sí mencionó en ese trabajo que su prueba daba también una cota inferior para el número de primos en los intervalos $(n,2n]$ (donde $n \in \mathbb{N}$) y que dicha cota es prácticamente la que predice el teorema de los números primos.

Recordemos que lo que Erdös hace en su artículo es acotar inferior y superiormente los coeficientes binomiales $c_{n}:=\binom{2n}{n}$ y comparar entre sí sendas estimaciones. La estimación inferior es $$\frac{4^{n}}{2n} \leq \binom{2n}{n}$$ y la obtiene de la identidad $\binom{2n}{0} + \ldots + \binom{2n}{2n} = 2^{2n} = 4^{n}$. La estimación superior la obtiene al estudiar de una manera muy astuta la descomposición en números primos de $\binom{2n}{n}$: en efecto, del teorema fundamental de la aritmética, de la fórmula Legendre (cf. A. M. Legendre, Essai sur la théorie des nombres. Seconde édition, 1808, págs. 8-10; de acuerdo con W. Narkiewicz, la atribución a de Polignac (1826-1863) y/o a Chebyshev (1821-1894) de este resultado es incorrecta) y de la desigualdad de Erdös-Kalmár se llega a que \begin{eqnarray*} \binom{2n}{n} &\leq & \prod_{p \leq \sqrt{2n}} p^{\alpha_{p}(c_{n})} \prod_{\sqrt{2n} < p \leq \frac{2}{3}n} p \prod_{n < p \leq 2n} p\\ &\leq& \prod_{p \leq \sqrt{2n}} (2n) \cdot 4^{\frac{2}{3}n} \cdot \prod_{n < p \leq 2n} p\\ &\leq& (2n)^{\sqrt{2n}} \cdot 4^{\frac{2}{3}n} \cdot \prod_{n < p \leq 2n} p \end{eqnarray*} para cada número natural $n \geq 3$. La conexión con el estudio de los números primos en $(n,2n]$ se acaba de hacer más que patente en este momento, ¿cierto?

De ambas estimaciones se desprende que si $n\geq 3$ entonces \begin{eqnarray*} 4^{\frac{n}{3}} \leq (2n)^{1+\sqrt{2n}} \prod_{n < p \leq 2n} p. \end{eqnarray*} De esto y de la desigualdad $$(2n)^{1+\sqrt{2n}} < 2^{\frac{n}{2}},$$ la cual es válida para todo número natural $n$ suficientemente grande (lo que en este caso quiere decir, en números redondos, siempre que $n > 22 \, 620$), se obtiene que $$\prod_{n < p \leq 2n} p > 2^{\frac{n}{6}}$$ si $n$ es suficientemente grande: ergo, para cada $n$ así de grande se cumple que $$(2n)^{\pi(2n)-\pi(n)} > 2^{\frac{n}{6}},$$ o equivalentemente que \begin{eqnarray*} \pi(2n)-\pi(n) > \frac{\log 2}{6} \cdot \frac{n}{\log 2n}. \end{eqnarray*} En resumidas cuentas: ¡la formulación clásica del postulado de Bertrand es sumamente conservadora!

## miércoles, 26 de abril de 2017

### A play on the interplay 'twixt primes and polynomials

In what follows, we shall denote the set of positive prime numbers by $\mathbf{P}$.

Act I. It is more or less well-known that there does not exist a non-constant polynomial $f \in \mathbb{Z}[x]$ such that $f(n) \in \mathbf{P}$ for every $n \in \mathbb{N}$. This can be proven by reductio ad absurdum: if $f(n) \in \mathbf{P}$ for every $n \in \mathbb{N}$ and $f(1) =: p$, then $p \mid f(1+kp)$ for every $k \in \mathbb{N}$; it follows that at least one of the equations $f(x)=p$ or $f(x)=-p$ has more solutions than $\deg(f)$, Q. E. A. This result is typically attributed to Christian Goldbach: W. Narkiewicz, on page 25 of his "The Development of Prime Number Theory", even mentions that it can be found in a letter from Goldbach to Euler written on September 28th, 1743. Luckily for us, Springer-Verlag published two years ago a translation into English of the correspondence of L. Euler with C. Goldbach edited and commented by F. Lemmermeyer and M. Mattmüller.

Act II. Several years ago, while perusing a very interesting article on primes in arithmetic progressions by M. R. Murty, I learned the notion of prime divisor of a polynomial: if $p \in \mathbf{P}$ and $f \in \mathbb{Z}[x]$, we say that $p$ is a prime divisor of $f$ if $p \mid f(n)$ for some $n \in \mathbb{Z}$. According to Murty, the basic theorem on the set of prime divisors of a non-constant $f \in \mathbb{Z}[x]$ can be traced back (at least) to a 1912 paper of I. Schur. The theorem can be proven emulating the celebrated proof of Eucl. IX-20.

Theorem. If $f$ is a non-constant polynomial of integer coefficients, then its set of prime divisors is infinite.

Proof. If $f(0)=0$, then every $p \in \mathbf{P}$ is a prime divisor of $f$. If $f(0) = c \neq 0$, then $f$ has at least one prime divisor as it can take on the values $\pm 1$ only finitely many times. Given any finite set $\mathcal{P}_{k}:=\{p_{1}, \ldots, p_{k}\}$ of prime divisors of $f$, we are to show that we can always find another prime divisor of $f$ which does not belong to $\mathcal{P}_{k}$. Let $A := p_{1} \cdots p_{k}$ and consider the equality $f(Acx)=cg(x)$ where $g \in \mathbb{Z}[x]$ is a polynomial of the form $1+c_{1}x+c_{2}x^{2}+\cdots$ where every $c_{i}$ is a multiple of $A$. Since $g$ is a non-constant polynomial whose constant term is different from $0$, $g$ has at least one prime divisor $p$. Clearly enough, this prime number $p$ is also prime divisor of $f$ which does not belong to $\mathcal{P}_{k}$. Q.E.D.

Act III. Resorting to the ideas in the previous paragraphs plus Dirichlet's glorious theorem on primes in arithmetic progressions, we are going to determine all the non-constant polynomials $f \in \mathbb{Z}[x]$ such that $f(\mathbf{P}) \subseteq \mathbf{P}$.

- If $f$ is one such polynomial and $f(0)=0$, then $f(x)=xg(x)$ for some some $g \in \mathbb{Z}[x]$. Given that $p \cdot g(p) =f(p) \in \mathbf{P}$ for every $p \in \mathbf{P}$, it follows that $g(p) = \pm 1$ for every $p \in \mathbf{P}$; therefore, in this case we obtain that either $g(x)=1$ and $f(x) =x$ or $g(x)=-1$ and $f(x)=-x$.
- Let us assume now that $f$ is one such polynomial and $f(0)=c \neq 0$. By the above theorem, we may fix a prime divisor $q$ of $f$ which is greater than $|c|$. If $n \in \mathbb{Z}$ is a witness of the fact that $q$ is a prime divisor of $f$, then $q \nmid n$. Thus, if $p_{1} < p_{2} < p_{3} < \ldots$ are all the positive primes in the arithmetic progression whose first term is $n$ and whose common difference is $q$, we have that $f(p_{i}) \equiv f(n) \equiv 0 \pmod{q}$ for every $i \in \mathbb{N}$, which is decidedly absurd because $f$ can assume the values $\pm q$ only finitely many times.

Hence, $f(x)=x$ is the only non-constant polynomial with integer coefficients which sends $\mathbf{P}$ to one of its subsets.

THE END.

## martes, 18 de abril de 2017

### Nunca hubo milagro

NUNCA HUBO MILAGRO

Banach y Tarski se encontraban gesticulando y argumentando, en el mismo cubículo, frente a un inmenso pizarrón verde, cuando demostraban el teorema que sería conocido a la postre como la Paradoja de Banach-Tarski: dada una bola sólida en $\mathbb{R}^{3}$, existe una descomposición de esta en un número finito de subconjuntos disjuntos que se pueden juntar otramente para producir dos copias idénticas a la bola original. Justo cuando terminaron la prueba, ambos callaron y se miraron muy contentos. Tarski hizo una pequeña aspiración y retuvo el aire un instante hasta que finalmente, abstraído, le dijo a Banach: "Ahora sabemos cómo fue que Cristo multiplicó los peces y el pan".

Autor: Enrique Ruiz.

Postdata. Leí este cuento por vez primera en 2016: no obstante, debo de confesar que estuve aguardando su aparición desde aproximadamente el primer semestre de 2004 pues fue más o menos por aquel tiempo que el Prof. Vulfrano T. me comentó que la multiplicación de los panes y los peces se podía conectar con el Axioma de Elección.

## sábado, 7 de enero de 2017

### Para la reflexión

«It is related of the Socratic philosopher Aristippus (c. 435 – c. 356 BCE) that, being shipwrecked and cast ashore on the coast of the Rhodians, he observed geometrical figures drawn thereon, and cried out to his companions: "Let us be of good cheer, for I see the traces of man." With that he made for the city of Rhodes, and went straight to the gymnasium. There he fell to discussing philosophical subjects, and presents were bestowed upon him, so that he could not only fit himself out, but could also provide those who accompanied him with clothing and all other necessaries of life. When his companions wished to return to their country, and asked him what message he wished them to carry home, he bade them say this: that children ought to be provided with property and resources of a kind that could swim with them even out of a shipwreck...»

(Vitruvio en De architectura [Libro VI])

## viernes, 14 de octubre de 2016

### Yet another function-theoretic proof of the Fundamental Theorem of Algebra

Let $f$ be a nonconstant polynomial with complex coefficients. Since $|f(z)| \to \infty$ as $z \to \infty$, we guarantee the existence of $R>0$ such that $$|f(z)|>|f(0)| \quad \quad (\ast)$$ for every $z \in \mathbb{C} \setminus \mathrm{B}_{R}(0)$. On the other hand, the continuity of the function $F \colon \overline{\mathrm{B}_{R}(0)} \to \mathbb{C}$ given by $z \overset{F}{\longmapsto} |f(z)|$ and the compactness of $\overline{\mathrm{B}_{R}(0)}$ allow us to ascertain the existence of $z_{0} \in \overline{\mathrm{B}_{R}(0)}$ such that $$|f(z_{0})| \leq |f(z)|$$ for every $z \in \overline{\mathrm{B}_{R}(0)}$. From $(\ast)$ we infer that $z_{0}$ is actually an element of $\mathrm{B}_{R}(0)$; then, by resorting to the Minimum-Modulus Principle, we conclude that $|f(z_{0})|$ must be equal to $0$ and we are done.

Scholia. a) If I understand correctly, the basic idea in this approach to the Fundamental Theorem of Algebra can be traced back to a 1748 memoir of d' Alembert. Yet, according to what we read in Reinhold Remmert's essay on the Fundamental Theorem of Algebra in [1, pp. 99-122], there were some gaps in d' Alembert's argument therein that would be pointed out by a twenty-two-year-old Gauss in the beginning of his doctoral thesis "Demonstratio nova theorematis omnem functionem algebraicam rationalem integram unius variabilis in factores reales primi vel secundi gradus resolvi posse", which he submitted to Pfaff at the University of Helmstedt in 1799 and through which he obtained his doctorate. However, it is noteworthy that, on that occasion, "... Gauss also [remarked], almost prophetically (Werke 3, p.11): 'For these reasons I am unable to regard the proof by d' Alembert as entirely satisfactory, but that does not prevent, in my opinion, the essential idea of the proof from being unaffected, despite all objections; I believe that ... a rigorous proof could be constructed on the same basis.'"
b) Interestingly enough, the proof of the Fundamental Theorem of Algebra showcased by Aigner & Ziegler's in their Proofs from THE BOOK (5th. edition, pp. 147-149) is based on the aforementioned d'Alembertian attack as subsequently simplified by Argand in 1814.

References
[1] H. D. Ebbinghaus, et al., Numbers. Graduate Texts in Mathematics 123, Springer-Verlag, NY, 1991.

## jueves, 21 de mayo de 2015

### Some very interesting paragraphs on the axiomatic method and its connection to some glorious moments in American History

«In the time of Euclid, and for over two thousand years thereafter, the postulates of geometry were thought of as self-evident truths about physical space; and geometry was thought of as a kind of purely deductive physics. Starting with the truths that were self-evident, geometers considered that they were deducing other and more obscure truths without the possibility of error. (Here, of course, we are not counting the casual errors of individuals, which in mathematics are nearly always corrected rather promptly.) This conception of the enterprise in which geometers were engaged appeared to rest on firmer and firmer ground as the centuries wore on. As the other sciences developed, it became plain that in their earlier stages they had fallen into fundamental errors. Meanwhile the "self-evident truths" of geometry continued to look like truths, and also continued to seem self-evident.

With the development of hyperbolic geometry, however, this view became untenable. We then had two different, and mutually incompatible, systems of geometry. Each of them was mathematically self-consistent, and each of them was compatible with our observations of the physical world. From this point on, the whole discussion of the relation between geometry and physical space was carried on in quite different terms. We now think not of a unique, physically "true" geometry, but of a number of mathematical geometries, each of which may be a good or bad approximation of physical space, and each of which may be useful in various physical investigations. Thus we have lost our faith not only in the idea that simple and fundamental truths can be relied upon to be self-evident, but also in the idea that geometry is an aspect of physics.

This philosophical revolution is reflected, oddly enough, in the differences between the early passages of the Declaration of Independence and the Gettysburg Address. Thomas Jefferson1 wrote:

"... We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness..."

The spirit of these remarks is Euclidean. From his postulates, Jefferson went on to deduce a nontrivial theorem, to the effect that the American Colonies had the right to establish their independence by force of arms.

Lincoln spoke in a quite different style:

"Fourscore and seven years ago our fathers brought forth on this continent a new nation, conceived in liberty and dedicated to the proposition that all men are created equal."

Here Lincoln is referring to one of the propositions mentioned by Jefferson, but he is not claiming, as Jefferson did, that this proposition is self-evidently true, or even that it is true at all. He refers to it merely as a proposition to which a certain nation was dedicated. Thus, to Lincoln, this proposition is a description of a certain aspect of the United States (and, of course, an aspect of himself). (I am indebted for this observation to Lipman Bers.)

This is not to say that Lincoln was a reader of Lobachevsky, [János] Bolyai or Gauss, or that he was influenced, even at several removes, by people who were. It seems more likely that a shift in philosophy had been developing independently of the mathematicians, and that this helped to give mathematicians the courage to undertake non-Euclidean investigations and publish the results.

At any rate, modern mathematicians use postulates in the spirit of Lincoln. The question whether the postulates are "true" does not even arise. Sets of postulates are regarded merely as descriptions of mathematical structures. Their value consists in the fact that they are practical aids in the study of the mathematical structures that they describe...»

I've excerpted these paragraphs (emphasis in bold was mine) from:

EDWIN E. MOISE, Elementary geometry from an advanced standpoint. Addison-Wesley Publishing Company, Inc. Second Printing, March 1964, USA, pp. 382-383.

Incidentally, as I was browsing through some of the past volumes of The American Mathematical Monthly the other day, I found on page 776 of the eighth issue of Vol. 99 of that periodical a letter from an Alberto Guzmán (Dept. of Mathematics, City College of CUNY) to the Monthly Editors wherein Mr. Guzmán mentions that it was Alvin Hausner the one who called his attention to the fact that the change in viewpoint, “from accepting axioms as obvious truths to stipulating them as working assumptions”, was reflected in the Declaration of Independence and the Gettysburg Address. Mr. Guzmán wrote that letter because, in the first issue of the said volume of the Monthly, there appeared an article by Abe Shenitzer that touched upon the nineteenth-century change of standpoint in question and it, presumably, refreshed his memory on what Hausner had told him about the matter once. It has to be noted, however, that in the missive there was no mention whatsoever to either Lipman Bers or the paragraphs by Edwin Moise showcased above: the corollary being that even the Monthly Editors nod off sometimes.

The aforecited excerpts are also interesting because it is known that Lincoln was at some point in his life an avid reader of Euclid. In point of fact, some of his phrases—such as "dedicated to the proposition" in the Gettysburg Address—are attributed to his reading of Euclid. In addition, Lincoln is said to have spoken once thus2:

"... In the course of my law-reading I constantly came upon the word demonstrate. I thought, at first, that I understood its meaning, but soon became satisfied that I did not. I said to myself, 'What do I do when I demonstrate more than when I reason or prove? How does demonstration differ from any other proof?' I consulted Webster's Dictionary. That told of 'certain proof,' 'proof beyond the possibility of doubt;' but I could form no idea what sort of proof that was. I thought a great many things were proved beyond a possibility of doubt, without recourse to any such extraordinary process of reasoning as I understood 'demonstration' to be. I consulted all the dictionaries and books of reference I could find, but with no better results. You might as well have defined blue to a blind man. At last I said, 'Lincoln, you can never make a lawyer if you do not understand what demonstrate means;' and I left my situation in Springfield, went home to my father's house, and staid there till I could give any proposition in the six books of Euclid at sight. I then found out what 'demonstrate' means, and went back to my law studies."

The following comments by Salomon Bochner in “The Role of Mathematics in the Rise of Science” (Princeton University Press, 4th printing, Princeton NJ, USA, 1981, p. 37.) provide us with additional references on Lincoln's interest in Euclidean geometry:

“... Abraham Lincoln, in his campaign biography of 1860, written by himself and published under the name of John L. Scripps of the Chicago Press and Tribune, ventured to assert about himself that 'he studied and nearly mastered the six [sic] books of Euclid since he was a member of Congress.' (The Collected Works of Abraham Lincoln, The Abraham Lincoln Association, Springfield, Illinois (Rutgers University Press, 1953), IV, 62.) Lincoln's assertion that he had 'nearly mastered' these books was one of the boldest and blandest campaigns statements in the annals of the American presidential elections, and folkloristic embellishments of this assertion were even less restrained. (See Herndon's Life of Lincoln (The World Publishing Company, 1949); Carl Sandburg, Abraham Lincoln, The Prairie Years (Harcourt, Brace & Co., 1926), I, 423-424; Emanuel Hertz, Lincoln Talks (Viking, 1936), p. 18.) It is worth reflecting on the fact that in the America of 1860 a consummate grassroots politician of the then Mid-Western Frontier should have thought that adding to a mixture of log cabin and rail-splitting a six books worth of Euclid would make the mixture more palatable to an electorate across the country.”

Last but not least, I would like to add that Lincoln's devotion to Euclid was exploited in a scene of Steven Spielberg's 2012 movie on the Great Emancipator. As the Hindu mathematician Bhāskara would say (or so the legend has it), BEHOLD! 3

P.S. Please, feel free to enter below any observation, suggestion, criticism, etc. you may have for the owner/writer of this blog...

______________
1 Jennnifer Schuessler (July 3, 2014). If only Thomas Jefferson could settle the issue (A period is questioned in the Declaration of Independence). The New York Times, p. A1.

2 Rev. J. P. Gulliver (September 4, 1864). Mr. Lincoln's early life: How he educated himself. The New York Times.

3 Be warned, though, that the short speech which Spielrock's Lincoln speaks in this scene is inaccurate in two or three respects.