## Sunday, May 27, 2018

### There is math even in Improv

From some time now, I have been immersing myself into the world of Improv. It is a great practice and it is not only a good distraction, but it also provides some great life lessons.

One of the important skills in Improv is to be in the moment. Several games in improv emphasize this, and one of them is called Enemy/Defender or Assassin.

In a group of people, everyone secretly chooses an enemy, whom they have to avoid, and a defender, whom they need to keep between them and their enemy. This defines the game dynamics. From a mathematical perspective, this defines a two dimensional dynamical system.

Each person is represented by a point. Each point has an enemy and a defender. Each point has to move in order to put its defender between them and their enemy. This can be modeled by $n$ points, with $n\geq 3$. The positions of the points can be taken to be vectors $P_i=(x_i,y_i)$. Associated with each $i$, there is an enemy and a defender $j$ and $k$, with $j\neq i\neq k$, with position vectors $E_i$ and $D_i$ respectively.

The goal then can be stated as having $D_i$ between $P_i$ and $E_i$, for all $i$. The condition of motion depends whether $P_i$ is on the ray $\overrightarrow{E_iD_i}$. This can be measured by the distance between $P_i$ to the line $E_iD_i$ and the projection of $\overrightarrow{P_iD_i}$ onto $\overrightarrow{E_iD_i}$.

Then, the dynamics of the system can be obtained by moving each point $P_i$ in order to satisfy these conditions on small time frames. It is important to avoid collisions and to restrict the $P_i$ to be only in the room to represent a more realistic situation.

This is a simulation in which for every $P_i$, a change in position $v_i$ is computed in terms of all the positions $\{P_1,P_2, \dots, P_n\}$ together with $E_i$ and $D_i$. Here, the initial positions and the enemy/defender arrangement is randomly generated, giving different behaviors depending on the initial configurations. The dynamics also depend on the parameters of the $v_i$, such as size and direction. Just changing these by a little could change the entire dynamics for the system. This is to be expected, as the stability of orbits are very sensitive to initial data (eg. Irrational rotations, Billiard dynamics, etc.)

Overall, Improv showed to be a great activity and even a nice excuse to use dynamical systems. Being in the moment and stable sometimes mean to avoid irrationality and incommensruable lengths. (Pun intended)

## Wednesday, November 29, 2017

Hace unos días, Sergio Lopez-Permouth posteó un meme sobre por qué $\pi=4$

Este es un ejemplo interesante de por qué algunos procesos no son completamente continuos cuando realizamos un límite. Un resultado similar muestra que $\sqrt{2}=2$,

En ambos casos el problema principal ocurre al querer aproximar la longitud de arco por medio de segmentos infinitesimales verticales y horizontales. En este caso tenemos que este no es un sistema lineal, sino que cuadrático. Es decir,

$$ds\neq dx+dy\,,$$

sino que

$$ds^2=dx^2+dy^2\,.$$

En otras palabras, siempre tendremos triángulos infinitesimales y es por esto que se obtiene una aparente paradoja visual.

Dada que esta relación sí se cumple en orden cuadrático, es de esperar que el resultado sea verdadero, no acerca de los perímetros, sino sobre las áreas, como bien lo sugirió Mario Blanco. Esto es cierto dado que al remover los cuadrados se obtienen una suma de Riemann correspondiente al área del círculo.

Si nos enfocamos en un cuadrante de la figura, por ejemplo

$$0\leq x\leq 1/2\,,y=\sqrt{1/4-x^2}\,,$$

tenemos que estamos realizando una suma de Riemann para la región entre

$$x=1/2\,, y=1/2\,, y=\sqrt{1/4-x^2}\,.$$

Es de notar que luego de la segunda etapa, los cuadrados obtenidos en la $n$-ésima etapa no tendrán todos el mismo lado. Por lo tanto es un tanto complejo escribir una serie que describa la suma de Riemann, sin embargo, sabemos que los lados de los cuadrados tienden a 0 cuando $n$ tiende a infinito, y esto asegura la existencia del límite y que sea igual a $1/4-\pi/4$.

Inicialmente quise escribir la serie, pero no me gustaría tener 20 páginas de ecuaciones, hay series más bonitas que describen $\pi$.

## Wednesday, January 25, 2017

### The voodoo behind 1+2+3+4+...=-1/12

A couple days ago, my friend Adolfo posted a rant on Facebook about the famous (or infamous) series

$$\sum_{n=1}^\infty n=-\frac{1}{12}\,.$$

His comment was not about the result itself, but about the little effort made by mathematicians to clarify this voodoo.

This result is very counter-intuitive as it states that if you add all positive integers, you will not only get a negative quantity, but also a fraction! I remember speaking about this result a couple years ago with a friend who was skeptical about it and he was telling me that it shouldn't be true since if he grabbed a calculator and start adding 1+2+3+4+..., he wouldn't approach a negative number, on the contrary, it would become bigger and bigger!

The trick lies in realizing we are talking about a mathematical object called a series, which is essentially different that any usual finite sum. One thing that can help to see that series are different than just adding a finite number of terms is that sometimes the order in which we add makes a difference! This is called Riemann rearrangement theorem, and states that if a series is conditionally convergent, then we can permute its terms to make it converge to any real number, or to make it diverge.

There are several concepts that play important roles here. One of them is the idea of dealing with an infinite number of things. It is not natural to operate with an infinite number of objects and a major problem is that we cannot directly apply an algorithm to compute this, as one of the key things of algorithms is that they must end, in other words, they must have a finite number of steps.

Hence, the traditional addition algorithm that we use in everyday life (or at least that our calculators use), cannot be applied directly to infinite series. Then the relevant question regarding

$$\sum_{n=1}^\infty n$$

is not how much is it? but rather, what is it?

The traditional approach for computing infinite series is by means of sequences of partial sums. This means that since we don't know what does it mean to add an infinite number of things, we approach infinity in the potential infinity sense, which establishes that infinity is the ability of taking an increasing sequence of big numbers forever. In other words, we take a big number of terms, we add them, and we assume that this result should somehow be close to the value of the series. Then we take a bigger amount of terms, and a bigger, and we keep doing this. Eventually, if the results obtained from the finite sums tend to cluster around a value, we say that that value is the result of the infinite series.

It is important to remark that when we write

$$\sum_{n=1}^\infty\frac{1}{2^n}=1\,,$$

we really don't mean equal in the same sense we mean 1+1=2. This first equal sign really means

$$\lim_{k\to\infty} s_i=1\,,$$

where

$$s_i=\frac{1}{2}+\frac{1}{4}+\dots+\frac{1}{2^i}\,.$$

Of course writing $\sum 1/2^n=1$ is shorter and conveys the same idea, or at least that's what lazy mathematicians think.  Again, the equal sign used here should not be thought as a comparison between two objects but rather as assigning a value to the series.

This assignment is very intuitive and we could say is very natural to think that this is a reasonable way to approach defining the meaning of an infinite series. But this it's not the only option. Deeper questions arise when this method of partial sums don't provide an answer, as it is the example of $\sum n$. The easy way out is simply to say there is no answer and the series diverges. But just as it happened with the equation $x^2=-1$, mathematicians saw an opportunity to extend the theory and also assign values to series for which the partial sums method is not enough.

It is possible to use the idea of actual infinity instead of potential infinity in order to approach these objects. An actual infinity approach considers having all infinite terms at the same time, as opposed to just a never-ending source of terms. Euler and Leibniz first started to develop ideas around divergent series and one of their key insights was to look at the meaning of a sum rather than its value.

A very interesting example of this occurs with the series

$$\sum_{n=0}^\infty (-1)^n\,.$$

This is an alternating sum for which the traditional approach of partial sums gives no answer (the partial sums jump from 0 to 1 and back forever).  But if we look a more abstract realization of the series, we can make it correspond to a geometric series,

$$\sum_{n=0}^\infty r^n\,.$$

This is another representation of the function $1/(1-r)$ inside the unit circle. With this we can make the conceptual assignment of $r=-1$ to this series and make the correspondence

$$\sum_{n=0}^\infty (-1)^n\mapsto \frac{1}{2}\,.$$

Sometimes we abuse of the notation and we prefer to write it down as $\sum(-1)^n=1/2$, but we have to stress the fact that "=" does not represent a comparison between objects but rather a correspondence

Something similar happens with the series $\sum n$. One of the most used methods involve the famous Riemann Zeta function. This is defined as

$$\zeta(s)=\sum_{n=1}^\infty \frac{1}{n^s}\,,$$

for complex numbers $s$ with $\Re(s)$ bigger than 1. It is possible to extend the definition of the Zeta function to the entire complex plane using the reflection formula

$$\frac{\zeta(1-s)}{\zeta(s)}=\frac{2\Gamma(s)}{(2\pi)^s}\cos\left(\frac{\pi s}{2}\right)\,.$$

Using the series representation of the Zeta function suggest the correspondence

$$\sum_{n=1}^\infty n\mapsto\zeta(-1)\,.$$

Hence, we can assign the value of

$$\zeta(-1)=\zeta(2)\frac{2\Gamma(s)}{(2\pi)^s}\cos\left(\frac{\pi s}{2}\right)=-\frac{1}{12}$$

to our series. Assign, not evaluate.

## Wednesday, October 26, 2016

### Golden ratio, Fibonacci numbers, and sums of squares

Today in my Foundations of Arithmetic class we talked a bit about the Golden Ratio. This is a nice number that appears in many places in mathematics, art, and nature!

The defining property of the Golden Ratio is a self-replicating quality of rectangles: We say that a rectangle has a Golden Ratio, or that its dimensions are in a Golden Ratio if after taking away the biggest square possible, the remaining rectangle is proportional to the original one.

This means that a rectangle like above will satisfy this condition if
$$\frac{a+b}{a}=\frac{a}{b}\,.$$

Having this property means in particular that it is possible to carry out this trick forever, that is, since the leftover rectangle is similar to the original one, it is possible to take another square out and have a smaller leftover rectangle which is similar to the original one. Then, the process can be applied once more, and once again, indefinitely.

This recursion does not hold for other ratios. For example, if the ratio between the dimensions is rational, this process actually ends.

For example, for a rectangle with dimensions $11\times 19$ we have that the process ends after 7 iterations

That is, after 7 times we had no leftover rectangle! The light blue $1\times 1$ leftover was a square already, so it is not possible to repeat the process. This happens because $\frac{19}{11}$ is a rational number. In the case of the golden ratio, it is an irrational number,

$$\phi=\frac{1+\sqrt{5}}{2}\,,$$

so we can never finish the process, as there will always be a leftover rectangle. A nice pattern arises when performing this procedure which is surprisingly related to continued fractions. For the ratio $11\times19$ we have that its simple continued fraction expansion is

$$\frac{19}{11}=1+\cfrac{1}{1+\cfrac{1}{2+\cfrac{1}{1+\cfrac{1}{2}}}}\,,$$

or

$$\frac{19}{11}=[1;1,2,1,2]\,.$$

Notice that there is 1 $11\times 11$ square, 1 $8\times 8$ square, 2 $3\times 3$ squares, 1 $2\times 2$ square, and 2 $1\times 1$ square, the same as the continued fraction coefficients!

Looking at this more geometrically, the continuous fraction expansion tells a way to write down the area of the rectangle as a sum of squares, that is,

$$11\times 19=209=11^2+8^2+3^2+3^2+2^2+1^2+1^2\,.$$

This is also true if the dimensions of the rectangle are commensurable, were the coefficients of the continued fraction indicates the number of times a certain square is repeated. If we have a rectangle with dimensions $p\times q$ with $p/q$ is rational. Then these squares will have dimensions that are of the type $d=np-mq$ with $n,m$ natural numbers.

In the case of a rectangle with dimensions $\phi\times 1$ this process gives

$$\phi=1^2+(\phi-1)^2+(2-\phi)^2+(2\phi-3)^2+\dots\,,$$

which, not surprisingly enough, can be written using Fibonacci numbers

$$\phi=\sum_{n=0}^\infty \left(F_n \phi-F_{n+1}\right)^2\,.$$

In general, for any real $x>1$ we can do this trick. Let the continued fraction of $x$ be given by

$$x=[a_0;a_1,a_2,a_3,\dots]\,,$$

then we have that considering a rectangle with dimensions $1\times x$ gives a square decomposition

$$x=\sum_{n=0}^\infty a_0(b_n x-c_n)^2\,,$$

which is finite if and only if $x$ is rational. An interesting problem would be to study the properties of the sequences $\{b_n\}$ and $\{c_n\}$, but I'll leave that for another time.

## Tuesday, April 19, 2016

### Segunda ley de termodinámica para los enteros

La entropía es una manera de medir la interacción entre lo pequeño y lo grande. Básicamente calcula el número de maneras en que se pueden obtener las mismas características macroscópicas en un sistema variando las propiedades microscópicas.

Un estado con mayor entropía es un estado altamente incierto, puesto que existen varias maneras de obtener dicho estado a partir de configuraciones y la probabilidad de obtenerlo en cierta configuración es entonces muy reducida. Sin embargo, estados con baja entropía tienen gran cantidad de información, puesto que solamente hay un número reducido de formas de obtener dicho estado, así una configuración en particular es altamente probable de ocurrir.

Es posible utilizar estas ideas con los enteros positivos. Podemos pensar que los micro-estados corresponden a los factores de un número y el macro-estado al número en si. Así el número $3$ tiene solo un micro-estado: $3$, sin embargo el número $12$ tiene $4$ micro-estados: $12$, $2\times 6$, $3\times 4$, $2\times 2\times 3$.

Si un macro-estado tiene $P$ diferentes micro-estados igualmente probables, la entropía asociada se define como

$$S=\ln P\,,$$

de esta manera podemos decir que la entropía de $3$ es $S_3=\ln 1=0$ y $S_{12}=\ln 4\sim 1.39$.

En general, podemos definir la entropía de un número como

$$S_n=\ln k\,,$$

donde $k$ es el número de formas distintas de factorizar $n$. Cuando $n=p$ es un número primo tenemos que $S_p=0$. Acá se puede ver la entropía para los números del 1 al 100 y del 1 al 10,000

Con esto podemos calcular el valor esperado de la entropía de un número.  Para $N$ un natural, sea $m=\max S_n$ para $n\leq N$.  Podemos entonces calcular el valor esperado de la entropía para los números hasta $N$ como

$$\langle S \rangle_N =\sum_{k=1}^N \frac{f_k }{N}\ln k\,,$$

donde $f_k$ es el la cantidad de números $n\leq N$ tales que $S_n=\ln k$. Para valores entre 1 y 100 y para valores entre 1 y 10,000 tenemos que las gráficas del valor esperado de la entropía son

Al principio el valor esperado de la entropía tiende a crecer, sin embargo a partir de $N=8,000$ parece estabilizarse un poco. ¿Pudiéramos tener una segunda ley de termodinámica para los naturales?

## Tuesday, March 15, 2016

### Probabilities in a loaded die

A few days ago, my friend Jesse and I were talking about how he was going to use actual dice in his class to teach students about probability. He would put them to roll dice and prove that actually each face has about the same probability of coming up.

He told me later that he looked online for loaded dice, that is, dice that would favor some of the faces and not have a uniform distribution. After a while, we thought it would be a nice problem to design a loaded die and to somehow calculate the theoretical probabilities for each face.  This is a  rather complex problem, so I started to think on reasonable simplifications to get something sound.

The main idea is to load a die by changing the center of mass of it. This would definitely impact the chances of getting a particular face. Usually we get that a regular die has a probability of $1/6$ of displaying a given face based on symmetry arguments. But when talking about a loaded die things get loaded.

Instead of considering a 3D die, I tried to analyze the 2D case, by considering a unit square with uniform mass density $\sigma$ and with a point mass $\mu$ at a point $(a,b)$.  This will make the square to have its center of mass at

$$\left(\frac{a \mu +\frac{\sigma }{2}}{\mu +\sigma },\frac{b \mu +\frac{\sigma }{2}}{\mu +\sigma }\right)\,,$$

and not at the center of the square. With this, the symmetries of the square are broken and a new description of its behavior has to be made.

In order to figure out the probabilities, first we have to define what do we mean by the probability of getting a specific face. One way of doing this is by considering the frequentists point of view of the system: lets roll the die many times and calculate the probabilities as relative frequencies of the outcomes.

Being this a a posteriori point of view, analyzing the physics of such situation becomes the best approach.

We want to have the square freely rotating, landing on a flat line, and then recording which face comes up. In order to do this, we have to prescribe an angular velocity of the rotating square, the landing vertex, and the incidence angle. We can simplify the model by just considering the picture when the die lands on the line, forgetting about its spinning past.

The argument is that by ignoring air resistance, friction, and everything else a good physicist will ignore, the landing angular velocity the square will land with is just a scalar multiple of its original velocity in the air (using conservation of momentum). So a good toy model is starting with a vertex fixed on the line, with an initial angle $\theta_0$ and an initial angular velocity $\omega_0$.  Using Newton's second law, we have that the sum of all torques is equal to the moment of inertia times the angular acceleration of the square. That is

$$I\frac{d^2\theta(t)}{dt^2}=\sum \tau\,,$$

were the moment of inertia for the square in this configuration is given by

$$I=\frac{\sigma}{3}+\mu(a^2+b^2)\,,$$

and where we only have one torque acting on the square due to gravity. Putting these together, we have that the system is described by the ODE

$$\frac{d^2\theta(t)}{dt^2}=-\frac{g || r ||}{I}\cos\left(\theta(t)+\delta\right)\,,$$

with initial conditions $\theta(0)=\theta_0$ and $\theta'(0)=\omega_0$, and where $g$ is gravity, $r$ the position of the center of mass of the square, and $\delta$ its argument.

This is a nonlinear differential equation which can be numerically solved in mathematica. Solving this equation gives us the rotation angle of the square in time. If the angle reaches $\pi/2$, it means that it completely tilted over the next face, and we would have to make the same analysis again but now with new initial conditions $\Theta(0)=0$ and $\omega(0)=\kappa \theta'(T)$, where $T$ is the time that took the previous face to completely reach the next one and $\kappa$ is a constant describing the energy loss the square experiences when landing on the next face.

We can keep this process as long as the equation

$$\theta(T)=\frac{\pi}{2}$$

has positive real solutions and the number of times this procedure is carries over will tell us on which face the square is going to stop rolling.

This approach is completely deterministic, as given any pair of initial conditions $(\theta_0,\omega_0)$, we can calculate on which face the square is going to stop rolling. In order to introduce probabilities, we can asume a probability distribution on the initial angles and the initial velocities.

It is reasonable to pick a uniform distribution of the incidence angles $\theta_0$, so I choose a uniform probability distribution on $(0,\pi/2)$. As for the initial velocities, the argument relies on the system being cyclic.  It is true that all angular velocities are not equally probable, as when someone grabs an actual die the angular velocities would have a distribution centered towards a preferred value, but when considering the problem only from the landing perspective, what really matters is that the system presents an almost periodic behavior on the initial velocity.  So we can take a uniform distribution on $(0,\omega_{\text{max}})\,.$ We also have to take into account that the die could rol clockwise or anti-clockwise with uniform probability.

Taking these assumptions into account would make the final probability not to depend on how we roll the die, that is, we are making it up for the lack of the symmetry the system has.

Finally we have to account for the probability of the die landing on a particular vertex. In order to calculate this, we can think that the probability of it landing on a given vertex is proportional to the time this vertex is the closest one to the landing line, while the die is rotating in the air. We can use the moments of mass about the face, and then calculate the probability of a vertex to be the average of its adjacent faces.

Performing some numerical simulations, we get the following probabilities ($\sigma=1, \mu=100$)

 (a,b) 0 1 2 3 (0.5,0) 0.268748 0.34257 0.0461125 0.34257 (0.1,0.1) 0.468528 0.468551 0.031472 0.0314486 (0.5,0.5) 0.25 0.25 0.25 0.25

and here is the code I used in Mathematica (the last function gives a the probability vector {0,1,2,3}):

T[{a_, b_}] = {1 - b, a};

INC[a_, b_, \[Mu]_, \[Sigma]_, c_] := \[Sigma]/
3 + \[Mu] Norm[Nest[T, {a, b}, c]]^2;

Sol[a_, b_, \[Mu]_, \[Sigma]_, \[Omega]0_, \[Theta]0_, c_] :=
NDSolve[{\[Theta]''[
t] == -((9.8 Norm[CM[Nest[T, {a, b}, c], \[Mu], \[Sigma]]])/
INC[a, b, \[Mu], \[Sigma], c])
Cos[\[Theta][t] +
ArcTan[Nest[T, {a, b}, c][[2]]/
Nest[T, {a, b}, c][[1]]]], \[Theta][
0] == \[Theta]0, \[Theta]'[0] == \[Omega]0}, \[Theta][t], {t, 0,
100}];

Rol[a_, b_, \[Mu]_, \[Sigma]_, W_, A_, c_] := Module[{w, Flag, n, X},
X = Sol[a, b, \[Mu], \[Sigma], W, A, c];
w = (0.9) ((D[(\[Theta][t] /. X),
t]) /. (NSolve[(\[Theta][t] /. X) == \[Pi]/2, t]))[[1]][[1]];
Flag = NumericQ[w] && Positive[w];
n = c;
While[Flag,
n++;
X = Sol[a, b, \[Mu], \[Sigma], w, 0, n];
w = (((D[(\[Theta][t] /. X),
t]) /. (NSolve[(\[Theta][t] /. X) == \[Pi]/2, t]))[[1]][[
1]]) (0.9);
Flag = NumericQ[w] && Positive[w];
];
n
]

Fr[a_, b_, \[Mu]_, \[Sigma]_, \[Theta]0_, c_] :=
Table[Mod[Rol[a, b, \[Mu], \[Sigma], w, \[Theta]0, c], 4], {w, 0, 1,
0.01}]

Pr[a_, b_, \[Mu]_, \[Sigma]_, \[Theta]0_, c_] := Module[{temp},
temp = Fr[a, b, \[Mu], \[Sigma], \[Theta]0, c];
{Count[temp, 0]/Length[temp], Count[temp, 1]/Length[temp],
Count[temp, 2]/Length[temp], Count[temp, 3]/Length[temp]}
]

Pro[a_, b_, \[Mu]_, \[Sigma]_, c_] := Module[{temp},
temp = Table[
Pr[a, b, \[Mu], \[Sigma], \[Pi]/2 (i), c], {i, 0, 1, 0.1}];
Total[temp]/Length[temp]
]

M0[a_, b_, \[Mu]_, \[Sigma]_] = (\[Mu] b + \[Sigma]/2)/(
2 \[Mu] + 2 \[Sigma]);
M1[a_, b_, \[Mu]_, \[Sigma]_] = (\[Mu] a + \[Sigma]/2)/(
2 \[Mu] + 2 \[Sigma]);
M2[a_, b_, \[Mu]_, \[Sigma]_] = (\[Mu] (1 - b) + \[Sigma]/2)/(
2 \[Mu] + 2 \[Sigma]);
M3[a_, b_, \[Mu]_, \[Sigma]_] = (\[Mu] (1 - a) + \[Sigma]/2)/(
2 \[Mu] + 2 \[Sigma]);

C0[a_, b_, \[Mu]_, \[Sigma]_] = (
M0[a, b, \[Mu], \[Sigma]] + M1[a, b, \[Mu], \[Sigma]])/2;
C1[a_, b_, \[Mu]_, \[Sigma]_] = (
M1[a, b, \[Mu], \[Sigma]] + M2[a, b, \[Mu], \[Sigma]])/2;
C2[a_, b_, \[Mu]_, \[Sigma]_] = (
M2[a, b, \[Mu], \[Sigma]] + M3[a, b, \[Mu], \[Sigma]])/2;
C3[a_, b_, \[Mu]_, \[Sigma]_] = (
M3[a, b, \[Mu], \[Sigma]] + M0[a, b, \[Mu], \[Sigma]])/2;

Prob[a_, b_, \[Mu]_, \[Sigma]_] := (1/
2) (C0[a, b, \[Mu], \[Sigma]] Pro[a, b, \[Mu], \[Sigma], 0] +
C1[a, b, \[Mu], \[Sigma]] Pro[a, b, \[Mu], \[Sigma], 1] +
C2[a, b, \[Mu], \[Sigma]] Pro[a, b, \[Mu], \[Sigma], 2] +
C3[a, b, \[Mu], \[Sigma]] Pro[a, b, \[Mu], \[Sigma], 3]) + (1/
2) (C0[1 - a, b, \[Mu], \[Sigma]] Pro[1 - a, b, \[Mu], \[Sigma],
0] + C1[1 - a, b, \[Mu], \[Sigma]] Pro[1 - a,
b, \[Mu], \[Sigma], 1] +
C2[1 - a, b, \[Mu], \[Sigma]] Pro[1 - a, b, \[Mu], \[Sigma],
2] + C3[1 - a, b, \[Mu], \[Sigma]] Pro[1 - a,
b, \[Mu], \[Sigma], 3])[[{1, 4, 3, 2}]]

Prob[a, b, \[Mu], \[Sigma]]

## Saturday, February 6, 2016

### Integrales complejas y sumas divergentes

Como parte de mi investigación he estado estudiando formas de regularizar ciertas series que a primera vista resultan divergentes.  Para esto se han desarrollado muchas técnicas de regularización y yo me he enfocado en utilizar funciones zeta.

Actualmente estoy estudiando un problema relacionado con sumas de Tornheim-Mordell y comencé a utilizar técnicas de análisis complejo para resolver el problema. Básicamente las herramientas más poderosas se basan en el Teorema del Residuo y sus muchas versiones. Un pariente cercano de este resultado es la Transformada de Mellin, la cual resulta ser una Transformada de Fourier disfrazada.

En términos simplistas, lo que dicen estos resultados es que es posible evaluar una integral de contorno por medio de ver los residuos que quedan encerrados.  Desde un punto de vista más físico, es el equivalente al Teorema de Gauss. Esta interpretación física resulta muy interesante, pues da una intuición de qué cosas esperar. Por ejemplo, resulta natural esperar que exista algún tipo de conservación de energía o carga eléctrica.  En el caso de funciones de variable compleja, tomando en cuenta algunas consideraciones, es posible tener esta conservación. En el plano complejo, esta conservación se traduce en que la suma de los residuos de la función suman cero,

$$\sum_{a\in\text{Polos simples}} \text{Res}(f(z),a)=0\,.$$

Sin embargo, en funciones sencillas como $f(z)=1/z$ parece que esto no aplica. Esta aparente falla se debe a que estamos olvidándonos de lo que sucede en $z=\infty$. Las funciones también pueden tener un residuo en infinito, y en este caso el residuo resulta ser

$$\text{Res} (f(z),\infty)=\text{Res}\left(-\frac{1}{z^2}f(1/z),0\right)=\text{Res}\left(-\frac{z}{z^2},0\right)=-1\,.$$

Con esta idea en mente, es posible estudiar lo que ocurre con funciones más interesantes. Por ejemplo, consideremos la función

$$f(z)=\pi \csc(\pi z)\,.$$

Esta tiene polos simples para $z=n,n\in\mathbb{Z}$ con residuos

$$\text{Res }(\pi \csc(\pi z),n)=(-1)^n\,.$$

Al tratar de calcular el residuo en infinito,  tenemos que da $\pm\infty$, lo que intuitivamente querría decir que tenemos un polo de mayor orden en infinito. Por lo tanto, podemos hacernos un poco los locos y decir que

$$\text{Res }(\pi \csc(\pi z),\infty)=0.$$

Si calculamos la integral de $f(z)$ a lo largo de una recta vertical, podemos pensar que estamos calculando la integral cerrada que pasa por infinito del semiplano izquierdo.  (El contorno $L$ encerraría el semiplano izquierdo dada la convención de que el interior de una región siempre queda a la izquierda de la dirección en que se recorre el borde, es decir, la dirección anti-horaria es positiva). Como $f(z)$ no tiene residuo en infinito, podemos esperar que la integral sobre $L$ será finita siempre y cuando $L$ no pase por ninguno de los polos finitos $z=n$. Por ejemplo tomemos $L$ como la recta vertical $\Re(z)=1/3$.  Entonces

$$\frac{1}{2\pi i}\int_L f(z)dz=\frac{1}{2\pi i}\int _L \pi \csc(\pi z)dz=\frac{1}{2}\,,$$

donde la integral puede facilmente calcularse. Ahora podemos hacer uso de la interpretación anterior y pensar que la integral en realidad puede calcularse sumando los residuos encerrados dentro de la región determinada por $L$,

$$\sum_{n=0}^\infty (-1)^{-n}=\sum_{n=0}^\infty (-1)^n=\frac{1}{2}\,.$$

La serie de la izquierda se conoce como la Serie de Grandi y ¡claramente es divergente! Una forma sencilla de verlo es el criterio de Leibniz, sin embargo al regularizar esta suma por diversos métodos se obtiene el resultado de $1/2$. Leibniz estudió esta serie y fue el primero en introducir cierto rigor en su cálculo. Fue duramente criticado, sin embargo Euler lo apoyó y fue así que comenzó el estudio se series divergente.

Podemos decir que la divergencia de la serie de Grandi se refleja en el hecho de que $f(z)$ tiene un polo en infinito, sin embargo al ser un polo de orden mayor, aún resulta posible obtener un valor regularizado para la serie, no en el sentido usual de convergencia, sino en un sentido más profundo.