## Monday, January 25, 2010

### Exponential Series

This semester we started a course in Time Scales, which is an interesting generalization of the classic differential analysis. The idea of time scales is to provide a connection between the study of differential equations made on $\mathbb{R}$ and the study of difference equations on $\mathbb{Z}$.

This connection is made by taking a closed subset of $\mathbb{R}$ and start defining on it notions of a right and left derivatives, which are called the $\Delta$ and $\nabla$ derivatives.

On last week's class, one of my friends was talking about this definitions and also how would one define an integral using this time scales approach. As an example, he state the following integral

$\int_0^\infty e^{-\tau^2}\Delta \tau$

On the time scale $\mathbb{T}=\overline{\{q^\mathbb{Z}\}}$ for some $q>1$. This is just the closure of the set of all integer powers of $q$.

After dealing with the boring algebra involved, the previous integral has a value of

$(q-1)\sum_{n=1}^\infty (q^n+q^{-n})e^{-n^2}$

Finding the actual value of this expression boils down to calculate the value of

$\sum_{n=1}^\infty e^{an-n^2}$

This can be done by using the Jacobi Theta Function, which is given by

$\vartheta(z,w)=\sum_{n=-\infty}^\infty \exp(2\pi i nz+\pi i n^2 w)$

for $z\in\mathbb{C}$ and $w\in\mathbb{H}$. Thus letting $z=\frac{-ai}{2\pi}$ and $w=\frac{i}{\pi}$ gives the value that we are looking for. In this case, we have that

$\int_0^\infty e^{-\tau^2}\Delta \tau$
$=\frac{(q-1)}{2}(\vartheta(-ai/2\pi,i/\pi) +\vartheta(ai/2\pi,i/\pi) -2)$

This is a really simple problem at first sight, but it caught my attention the fact that it all relies on determining the value for an expression that looks like

$\sum_{n=1}^\infty e^{p(n)}$

where $p(n)$ was a quadratic polynomial with a negative leading coefficient. A natural question came then to my mind, what would happen if we would have any polynomial instead?

My first idea was to study the case when $p(n)=-n^m$ for a fixed power $m$. The cases when $m=1,2$ are contained in the previous approach using the Jacobi Theta Function. I first try to calculate the series for some values of $m$, and I found that the above expression, as a function of $m$, converged really fast as $m$ was getting bigger.

Actually, it is not difficult to find out that this limit exists and has the value of

$\lim_{m\to\infty} \sum_{n=1}^\infty e^{-n^m}=\frac{1}{e}$

Following the same line, one can show that for $p(n)=-an^m$, with $a$ a constant, we have that

$\lim_{m\to\infty} \sum_{n=1}^\infty e^{-an^m}=\frac{1}{e^a}$

Thus, one could say that for a polynomial $p(n)=-an^m+O(n^{m-1})$ a good approximation its given by

$\sum_{n=1}^\infty e^{p(n)} \approx\frac{1}{e^a}$

Looking for a different approach towards a more precise answer, one can define a function given by $f(t)=\sum_{n=1}^\infty e^{p(n) t}$ and this function can be viewed as the heat kernel of a differential operator $P$ whose spectrum is given by $\sigma(P)=\{\lambda_n\}$, where $\lambda_n=p(n)$.

This could suggest the difficulty of such a closed form for $f(1)$, for instance, in the case of $p(n)=-n^m$, this would be related to the existence of an operator $P$ with eigenvalues $\{1/n\}$, which is one of the consequences of the Riemann Hypothesis.

## Monday, January 11, 2010

### Hausdorffización de un Espacio?

El año pasado después de una plática sobre topología en la universidad, como cosa rara en un ambiente topologico, surgió una pequeña plática sobre espacios de Hausdorff. De dicha discusión, sirgió en mi la inquietud si de alguna forma pudiera medirse cuán Hausdorff es un espacio.

Un espacio topológico se dice Hausdorff si es posible separar puntos, es decir, si dados dos puntos distintos, se pueden hallar vecindades de cada punto disjuntas entre sí. Una definición más formal sería que dados dos puntos distintos $x,y$ de un espacio topológico $X$, existen abiertos $U,V\in \mathcal{T}$ tales que $x\in U$, $y\in V$ y $U\cap V=\emptyset$.

De cierto modo, dos puntos que fallan en cumplir esta propiedad están unidos, en el sentido que cualquier vecindad de uno intersecta a todas las vecindades del otro.

Siguiendo esta conceptualización, si tenemos que la propiedad de separación falla en dos puntos $x,y$, hablando de una manera informal podríamos decir que los puntos no pueden tener vecindades arbitrariamente pequeñas. Con esto en mente, se me ocurrió definir una función $\mathcal{W}:X\to \mathcal{P}(X)$ por $x\mapsto \bigcap_{V\in\mathcal{T}} U$.

La idea es hallar la menor vecindad alrededor de cada punto. Sin embargo, nótese que $\mathcal{W}_x$ no será necesariamente un conjunto abierto, de hecho, en el caso que se trate de un espacio Hausdorff, no es muy difícil de probar que dicha función $\mathcal{W}$ es simplemente $\mathcal{W}_x=\{x\}$ y por lo tanto, si el espacio es $T_1$, se tiene que $\mathcal{W}_x$ es cerrado.

Posiblemente, esta función pueda ayudar a medir en cierto modo que tan Hausdorff es un espacio. Si un espacio es Hausdorff, se tiene que $\mathcal{W}_x=\{x\}$ y sería natural preguntar si esta condición es suficiente para ser Hausdorff, es decir, un espacio $X$ es Hausdorff si y solo sí $\forall x\in X$, $\mathcal{W}_x=\{x\}$.

Supongamos que $X$ es un espacio tal que $\mathcal{W}_x=\{x\}$ para todo $x\in X$ y $X$ no es Hausdorff. Sean $x,y$ dos puntos distintos de $X$ tales que cualquier vecindad de $x$ intersecta a todas las vecindades de $y$.

Sea $\mathcal{B}_x$ una base local en $x$ y $\mathcal{B}_y$ una base local en $y$. Puesto que $U\cap V=\emptyset$ para todo $U\in\mathcal{B}_x$ y todo $V\in\mathcal{B}_y$, sea $l_{UV}\in U\cap V$. Utilizando el axioma de elección, defínase $L$ el conjunto de dichos $l_{UV}$. Por definición, se tiene que $x$ es un punto límite de $L$.

Supongamos que $X$ es segundo contable, de esta manera, es posible encontrar una sucesión $\{l_n\}$ en $L$ que converja a $x$. Por definición, para todo $n$, existe una vecindad $V$ de $y$ tal que $l_n\in V$, por lo tanto $l_n\in\overline{V}$ y entonces $x\in \cap_{y\in V\in \mathcal{T}} \overline{V}$ $=\overline{\cap_{y\in V\in \mathcal{T}} V}$ $=\overline{\{y\}}$. Si se supone además que $X$ es $T_1$, entonces $\overline{\{y\}}=\{y\}$, por lo que $x\in \{y\}$ y entonces $x=y$.

Al parecer, esta equivalencia es válida en el ámbito de $X$ ser $T_1$ y segundo contable, no se si puede ser generalizada esta noción, pero me pareció una forma interesante de traducir el concepto de Hausdorff en términos de la menor vecindad que contiene a un punto.

## Monday, January 4, 2010

### Line orbits on a circle

After a little break time, I decided that a good way to start the year is by posting something that I was thinking on few days ago.

At the beginning of the Winter break, one of my friends back home post me a question about determining the foci a hyperbola just with compass and straightedge constructions. Thinking a little about it, I was trying to find some basic property of the foci of a hyperbola, and it came to my mind that they must have the same property no matter what kind of conic we are looking at, since all conics are equivalent under the $PSL(2,\mathbb{R})$ group.

So I tried to find some kind of property that the foci of all conics share, and one of them can be regarded as some kind of reflection property.

If you have a set of lines that passes through one of the foci of a conic and you take the reflection of these lines on the conic, the resulting set of lines passes through the other focus.

In the case of a circle, the two foci coincide in the center of the circle, so the condition holds, but this made me think of analyzing the orbit of a line in a circle. By this, what I mean is the following: start with a line that intersects the circle, then at the intersection points, reflect the line through the circle and keep with this process. Call the resulting intersection points on the circle, the orbit of the line. Now, a very natural question would be for which kind of lines does this orbit is finite? has a limit point? is dense in the circumference?
At the beginning, the answer seems really straightforward, one could say that the lines that lead to finite orbits are the ones that belong to sides of a regular polygon, which means that the lines leading to finite orbits are the ones whose distance to the center of the circle equals the apothem of some regular polygon inscribed in the circle.

However, this condition is quite weak, since there are more such lines. To look at these, we can take a different approach. One easy way is to look this is by taking the arc length instead. If we take the length of the circumference to be 1, then if the arc lengths of the pieces into which is divided the circle by the line are rationals, we have that the orbit is finite. Moreover, if the arc length is $q/p$, the orbit has length $p$.

This is equivalent to studying the circle regarded as $\mathbb{R}/\mathbb{Z}$, and here the orbit for a value $x$ is the set $\{[nx]: n\in\mathbb{Z}\}$ from where we have that it is finite iff $x\in\mathbb{Q}$ and it is dense in the circle otherwise.

Another interesting question would be to find sufficient conditions for finite orbits on a general conic, and this might be studied using the group structure of the circle $S^1$ and the projective properties of $PSL(2,\mathbb{R})$.