## Monday, September 21, 2015

### The geometry of irrationals

Many years ago I remember reading about irrational numbers and the various surprising things they hide. One nice thing about irrationals is that even thought we think of them as being bad behaved and ugly, we can still classify them further. For example, we have irrationals that are also algebraic, that is, they are zeros of a polynomial with integer coefficients.

We can think that rationals are algebraic of degree 1, while algebraic of degree 2 include $\sqrt{2}, \sqrt{3}$, the golden ratio, etc.

Then we have numbers that are not algebraic, that is, they are not zeros of polynomials with integer coefficients. These are called Transcendental. Some of the famous ones in this category are $\pi, e$, Liouville's constant, etc.

Another way of studying irrational numbers is by measuring how irrational are they. This really means to study how well a number can be approximated with rational numbers, that is, loosely speaking, how fast the denominators of the fractions approximating the number have to grow to stay close to it.

We can think that this has to do with the representation of real numbers with approximations by rationals. In a more philosophical way, this irrationality measure can tell how well we can approximate numbers in the math world, with numbers in the real world.

Liouville was big into approximating irrationals with rationals trying to do it in the most efficient way, that is, controlling the size of the denominators. More recently, Roth made contributions into a deeper understanding of this, leading him to win the Fields Medal in 1958.

Looking at this Facebook post about the integer lattice in $\mathbb{R}^3$, I remember an attempt at defining another way of measuring the irrationality of a number.

First, consider the integer lattice in $\mathbb{R}^2$.  Given a real number $m$, graph the line $y=mx$.  Thus the line will hit the lattice if and only if $m$ is rational, so the idea is to analyze what happens when $m$ is irrational.

For this, consider the following: consider a radius $r>0$ and draw the biggest circular sector of radius $r$ that is symmetric around the line $y=mx$ and that does not contain any point of the lattice.

That is, the sector must not contain any of the blue points from the lattice.  Now, the idea is to measure this area and treat it as a function of the radius $A_m(r)$. I created a GeoGebra demonstration to show the idea of the sector.

Here you can see how the area of the sector changes when the radius changes, and also how the procedure varies when considering different values for $m$.

Effectively what happens is that we are looking for the best approximation of the number $m$ using a fraction $p/q$ with $p^2+q^2\leq r^2$. This way we control the size of both numbers for the approximation of $m$.

Thus we have that
$$A_m(r)=r^2\big|\arctan(m)-\arctan(f_m(r))\big|\,,$$
where $f_m(r)$ is the best approximation to $m$ with a rational inside the circle of radius $r$, that is
$$f_m(r)=\min \big|m-\frac{p}{q}\big|\,,\quad \text{ where }p^2+q^2\leq r^2\,.$$

Now, the trick is to analyze $A_m(r)$ as $r\to\infty$. One might think that this is trivially zero, since any irrational can be written as the limit of a sequence of rationals, but we also have the radius growing up to infinity, hence becoming an indeterminate form.

That is, it might be possible to propose another irrationality measure as

$$\nu(m)=\lim_{r\to\infty} \sup A_m(r)\,.$$

The $\sup$ just ensures the existence of $\nu(m)$, we don't even know if there is a limit!

If $m$ is rational, this is exactly zero, but the question would be what happens for $m$ irrational. I did some numerics to have an idea of this measure for a couple of cases. Of course this is no evidence, as a limit like this cannot be trivially calculated using numerical methods, but at least gives a bit of insight.

For example, for $m=\sqrt{2}$ we have that the graph of $A_{\sqrt{2}}(r)$ seems to oscillate between 0.2 and 1.4. We can see here that the peaks happen when a new rational approximation is found. Naively speaking, $\sqrt{2}$ is not hard to approximate with rationals, and hence we don't have to increase too much in $r$ to find a new approximation.

On the other hand, for $m=\pi$ we have a different behavior. We have a series of good approximations, but then suddenly we cannot find any more good approximations. Even though it looks like $A_\pi(r)$ went to 0 around $r=400$, if we zoom in we can see that it is slowly rising finding a new approximation.

I believe this is a very interesting topic and I think there is something nice waiting to be discovered here. Even if it leads to the same notion as the Liouville-Roth exponent, this would provide a more geometric interpretation of the behavior of irrational numbers.

## Thursday, September 10, 2015

### Con matemáticas no hay sorpresas

Para muchos, las recientes elecciones en Guatemala han resultado en una gran sorpresa debido a sus resultados. Sin embargo, con matemáticas no hay sorpresas.

Hace mas de un mes hice varias proyecciones utilizando los resultados de varias encuestas, tanto las principales, como las diversas encuestas encontradas en internet. Los resultados de las proyecciones en base a las principales encuestas publicadas en medios impresos arrojaron que

Comparación entre proyecciones y resultados actuales

en donde se tenía un error de aproximadamente 3%. Al analizar las proyecciones que incluían las encuestas realizadas por internet se encuentra que estas estaban muy sesgadas, aunque igualmente la tendencia de FCN al primer lugar y a una diferencia muy estrecha entre UNE y LIDER seguía observándose (menos de un 3%).

Participación en elecciones pasadas para 1era y 2nda vuelta

Otra de las grandes sorpresas fue la participación ciudadana. Al momento se calcula que ha sido del 71.32%, algo que no se había dado en la historia política de Guatemala. Sin embargo, al analizar la historia de todas las elecciones desde 1985 se puede observar una tendencia interesante.

Los datos en azul representan los porcentajes de participación en primera vuelta (incluyendo la del 2015) y los datos en rojo la participación en segunda vuelta. A partir de 1995 se nota que la tendencia a la participación ha ido en aumento casi constante, por lo que se puede realizar un análisis de valores atípicos para poder encontrar un modelo de regresión lineal que pueda describir los datos. Al realizar esto, se obtiene que los datos están correlacionados en 98.49% para la primera vuelta, y 95.98% en la segunda vuelta.

Comparación entre proyecciones y valores actuales

Con esto, se tenía que la proyección daba una participación del 72.39%, lo cual difiere en 1.07% del valor actualmente registrado. Así mismo, se espera que en segunda vuelta haya una participación de al rededor del 63.4%.

Al analizar datos de una forma sistemática y formal, no existen sorpresas.