## Deformed Numbers and Calculus

When I first heard of fractional calculus, I thought that it sounds contrived. But then again, so are many abstractions and generalizations of established ideas in mathematics. There is one good motivation for this idea, which strikes at the heart of fundamental physics i.e. the Dirac operator $\hat{D}$ which is understood as the square root of the Laplacian $\Delta$ i.e. $\hat{D}^2=\Delta$. If it is one dimensional with $\Delta=\partial_x^2$, this would have been straightforward. For more than one dimension, this is nontrivial – what is the square root of say $\partial_x^2 + \partial_y^2$?

The reason my attention got to this is through a visitor of mine, Prof. Won Sang Chung from Gyeongsang National University who visited the institute for the period of February 2-14, 2017. His research has been mainly on deformation of physical and mathematical structures. His interest in noncommutative quantum mechanics was the initial reason of our contact. With respect to the above topic, Prof. Won’s many works on deformation has led him to study the deformation of units for instance in the published work “On the q-deformed circular imaginary unit and hyperbolic imaginary unit: q-deformed rotation in two dimension and q-deformed special relativity in 1+1 dimension“. It is interesting that this work was carried out with an undergraduate, of which I’m told, work together with Prof. Won a few times a week at late evening hours. There are other works by them that touches on arithmetic operations of numbers, for which they call it $\alpha$-deformed numbers. In a way, it shifts the idea of deformation of a complex operation of say the derivative to arithmetic ones.

$x\oplus_f y=f^{-1}(f(x)+f(y))$

where $f$ are some specified function and $x,y$ are integers. By making $f(x)=x^\alpha$, one then has the alpha-deformed addition

$x\oplus_\alpha y= (x^\alpha + y^\alpha)^{1/\alpha}$

for $x,y>0$. For more cases of the integer, it is

$x\oplus_\alpha y = \vert \vert x\vert^{\alpha-1}x +\vert y\vert^{\alpha-1}y\vert^{\frac{1}{\alpha}-1} (\vert x\vert^{\alpha-1}x + \vert y\vert^{\alpha-1}y)\ .$

Suffice for our discussion to take the simpler case. It is easy to show that the additive identity is preserved

$x\oplus_\alpha 0=0\oplus_\alpha x= x$

The additive inverse requires the definition of $\alpha$-deformed subtraction:

$x\ominus_\alpha y = \begin{cases} (x^\alpha - y^\alpha)^{1/\alpha}\quad &(x>y)\\ -(y^\alpha - x^\alpha)^{1/\alpha}\quad &(x

Consecutive addition can be done easily. Now we can define the $\alpha$-deformed numbers by noting the following:

$0_\alpha=0,\quad 1_\alpha=1,\quad (-1)_\alpha=-1$

and

$(n)_\alpha = 1\oplus_\alpha 1\oplus_\alpha \cdots \oplus_\alpha 1=n^{1/\alpha}\ .$

The multiplication and division operations are taken as the normal ones (undeformed) and

$(mn)_\alpha = (m)_\alpha (n)_\alpha\quad;\quad \left(\cfrac{m}{n}\right)_\alpha=\cfrac{(m)_\alpha}{(n)_\alpha}\ .$

Thus, the $\alpha$-deformed numbers form a commutative ring and essentially there is a 1-1 correspondence between the $\alpha$-deformed numbers and the integers. By the division operation defined above, I guess one could easily extend these deformed integers to $\alpha$-deformed rational numbers.

The topic stopped there will not be too interesting. The authors went on to define the $\alpha$-deformed derivation to build up calculus. It is here, there is a leap of faith to say that the $\alpha$-deformed rational numbers can be completed to form $\alpha$-deformed real numbers. Suppose that this can be done, one defines the $\alpha$-derivative by

$D^\alpha_x F(x) =\lim_{y\rightarrow x}\ \cfrac{F(y)\ominus_\alpha F(x)}{y\ominus_\alpha x}\ .$

One can show indeed that this definition gives a derivation (obeying Leibniz rule) – see paper. Hence, one finds

$D^\alpha_x x\ominus_\alpha xD^\alpha_x = 1\quad\Rightarrow D^\alpha_x x= 1\oplus_\alpha xD^\alpha_x\ .$

So one can easily show that for example, $D^\alpha_x x^n= n_\alpha x^{n-1}$ i.e. it works like the ordinary derivative with numbers replace by the deformed numbers. One could generalise this further to power series functions. For instance the $\alpha$-exponential function defined by

$D^\alpha_x e_\alpha (x) = e_\alpha (x)\ .$

One can extend the deformed numbers to that of $\alpha$-deformed complex numbers $z_\alpha=x_\alpha\oplus_\alpha iy_\alpha$ and form the $\alpha$-trigonometric function via the complex $\alpha$-exponential function. In fact one can show that these functions obey the usual Euler relation.

Once we build up these function and their calculus, one can start to think about solving standard physics problems via $\alpha$-deformed differential equations. We will state only the case of the quantum harmonic oscillator which was worked out in the paper. The differential equation to solve is

$\left(-\cfrac{\hbar^2}{2m}(D^\alpha_x)^2\oplus_\alpha\cfrac{1}{2}m\omega^2 x^2\right)\ u= Eu\quad .$

Suffice for our discussions, to state the results for its spectra, namely

$E_n=2^{\frac{1}{\alpha}-1}\hbar\omega\left(n+\cfrac{1}{2}\right)^{1/\alpha}\$

Note that the energy levels almost retain its functional form but note that the energy level spacings

$E_{n+1} \ominus_\alpha E=2^{\frac{1}{\alpha}-1}\hbar\omega$

are only equidistant in the $\alpha$-deformed sense! So, there are nontrivialities associated with the earlier deformations of the arithmetic operations. In one of his talks, he had mentioned that the case of $\alpha$-deformed hydrogen atom has also been worked out with similar functional form of the spectra but of course again deformed. The applications that Prof. Won had mentioned are the case of quantum systems whose primary potential is known but there are missing information about them. He envisaged that by doing some fitting of the spectra, better physical understanding of previously unsolved systems (but spectrally known) can be achieved using these $\alpha$-deformed theories.

## Choosing Zeidler on QFT

Some weeks back just before the Singapore trip, it was suggested that I should do reading seminars on quantum field theory (QFT) and/or particle physics. A decision to be made is which book to be chosen. My first exposure to quantum field theory is through Herbert Green‘s course on “Elementary Field Theory” in Adelaide and as usual, his treatment is unique and usually has no text-book reference similar to what he is doing. The next course that had field theory in it was particle physics for which I referred to is T.D. Lee‘s “Particle Physics and Introduction to Field Theory“, which is clear enough for me though his space-time signature is (+1,+1,+1,+i). It was only later (after graduating), that I was introduced to Mandl & Shaw by my eldest brother, when he gave me the book as a present. Later, I used this as one of my main reference during my Part III course on Quantum Field Theory in Cambridge. At the time, however, I heard praises of the book by Itzykson & Zuber, which is of course harder to read but my guess contained a lot of gems (never finished reading it).

For our group, I thought I will have something different, tailored for a more mathematical-inclined audience. Even here, there are several choices. I settled for the thickest book I can find. Saw Zeidler with his intended six-volume book. Apparently, he did not quite finish the six volumes since he passed away in November 2016. Three volumes of “Quantum Field Theory – A Bridge Between Mathematicians and Physicists” were published. So we begin with Volume 1: Basics in Mathematics and Physics.

A Prologue was written to describe the style and scope of  the book with an outline of the 6 volumes. The book is filled with quotations and historical anecdotes, making it untiring to read. It opens with five different quotations of famous physicists  and mathematicians leading to the five golden rules of the book:

1. Write with an open landscape with depths of perspectives.
2. Teach the content with a battery of problems.
3. A technical book which is readable beyond the first several pages.
4. Delves into deep questions of physics interlinking with mathematics.
5. Teach for the appreciation of the physics.

The prologue also uses the often-quoted example of how successful QFT is in calculating the anomalous magnetic moment of the electron. Let us recall that any current-carrying loop produces a magnetic moment $\underline{M} = \frac{1}{2}\underline{r} \times\underline{j}$ where $\underline{j}$ is the current density of the loop. The current may be carried by a charged particle orbiting in a loop. From the formula given, the magnetic moment is then proportional to its angular momentum.

If one let the particle be an electron carrying only the spin angular momentum $\underline{S}$, then its magnetic moment is

$\underline{M}_e = \cfrac{g_e\mu_B\underline{S}}{\hbar}\ ,$

where $\mu_B=e\hbar/(2m_e)$ is the Bohr magneton and $g_e$ is the gyromagnetic factor (dimensionless magnetic moment). Now according to Dirac’s theory, $g_e=2$ but this quantity receives corrections from QFT which can be written as

$g_e=2(1+a)\ .$

The correction $a$ can be written as a (divergent) power series of the fine structure constant $\alpha$. Up to fourth power of $\alpha$ (using 891 Feynman diagrams), the correction is

$a_{th}=0.001 159 652 164 \pm 0.000 000 000 108\ .$

Experimentally it is

$a_{expt}=0.001 159 652 188 4\pm 0.000 000 000 004 3\ .$

This is indeed excellent agreement. The calculation does not stop there. The next fifth power of $\alpha$ correction involves 12,672 Feynman diagrams giving a correction which is accurate to more than 1 part in a billion. See the 2006 PRL paper here. The question now that this series that gave the correction is said to be divergent.

QFT is a practical theory which goes on to compute useful quantities for physicists:

• cross-sections of scattering processes of particles
• masses of stable elementary particles
• lifetime of unstable elementary particles

Thus one ultimate goal of QFT is to bring it a rigorous mathematical setting. Perhaps it is worth mentioning here the work of Kreimer that has connections to noncommutative differential geometry.

In the search for a mathematical framework, some guiding principles from mathematics are

1. Going from concrete structures to abstract ones
2. Combining abstract structures
3. Functor between abstract structures
4. Statistics of abstract structures

In the last one, much help was given by physics (Zeidler used the term physical mathematics which I will not adopt) through Feynman functional integrals which we will go into the coming readings.

## Geodesics on the Hyperbolic Plane

Continuing from the last post with the upper half plane model as our model of the hyperbolic plane. First, it is to be noted that the fractional linear transformations or more commonly called the Moebius transformations maps the upper half plane $H$ to itself.

Consider the transformed point under $T_g$ on $z\in H$ where $g=\begin{pmatrix} a&b\\c&d \end{pmatrix}$ :

$w\equiv T_g(z)=\cfrac{az+b}{cz+d}=\cfrac{ac\vert z\vert^2 + adz+bc\bar{z}+bd}{\vert cz+d\vert^2}$ .

The imaginary part of the transformed point is

$\textrm{Im}(w)=\cfrac{w-\bar{w}}{2i}=\cfrac{(ad-bc)(z-\bar{z})}{2i\vert cz+d\vert^2}=\cfrac{\textrm{Im}(z)}{\vert cz+d\vert^2}$ .

Note that if $\textrm{Im}(z)>0$ then the above shows that $\textrm{Im}(w)>0$ also, and hence the earlier statetment of the map onto $H$ itself.

To continue studying the geometry of the hyperbolic plane, one must have a notion of hyperbolic distance and this is obtained from the previous hyperbolic metric on the upper half plane. One thus defined the infinitesimal element of the hyperbolic distance to be

$ds=\cfrac{\sqrt{dx^2+dy^2}}{y}$ .

Given a curve $\gamma(t)=\{v(t)=x(t)+iy(t)\vert t\in H\}$, its length is thus given by

$h(\gamma)=\displaystyle\int\limits_0^1 \cfrac{\sqrt{\left(\frac{dx}{dt}\right)^2+\left(\frac{dy}{dt}\right)^2}}{y(t)}\ dt$ .

For the hyperbolic distance between $z,w\in H$, one takes the minimum value of the length of curves connectig the two points i.e.

$\rho(z,w)=\inf_\gamma h(\gamma)$ .

Later, it will be convenient for us to know exactly what are the shortest hyperbolic curves connecting two point on $H$ i.e. the geodesics. For now, we note that the Moebius transformations themselves are isometries (preserve lengths). Consider the transformed points on a curve $\gamma(t)$ :

$w(t) = T(z) = \cfrac{az+b}{cz+d} = u(t) + iv(t)$ .

Its derivative is

$\cfrac{dw}{dz}=\cfrac{a(cz+d)-c(az+b)}{(cz+d)^2}=\cfrac{1}{(cz+d)^2}$ .

Noting that $v=y/\vert cz+d\vert^2=\textrm{Im}(w)$, we can write the derivative as $\vert\frac{dw}{dz}\vert = v/y$. Thus under the transformation, the hyperbolic distance stays invariant as shown below:

$\begin{matrix} h(T(\gamma))&=\displaystyle\int\limits_0^1\cfrac{\vert\frac{dw}{dt}\vert\ dt}{v(t)}\\ &=\displaystyle\int\limits_0^1 \cfrac{\vert\frac{dv}{dz}\vert\vert\frac{dz}{dt}\vert\ dt}{v(t)} \\ &=\displaystyle\int\limits_0^1\cfrac{\vert\frac{dz}{dt}\vert dt}{y(t)}= h(\gamma) \end{matrix}$ .

Next, we claim that geodesics in $H$ are either (arcs of) semicircles and vertical lines orthogonal to the real axis. We calculate first, the formula of the hyperbolic length of two points on a semicircle with centre $(c,0)$ (see diagram below).

The two points can be coordinatized by the angles $\alpha,\beta$ or parametrized by the following:

$x=c+r\cos t\ ;\quad y=r\sin t$ .

Their differentials are

$dx=-r\sin t dt\ ;\quad dy= r\cos t dt$ .

Plugging into the distance formula on a hyperbolic curve gives

$\rho(\alpha,\beta)=\displaystyle\int\limits_\alpha^\beta \cfrac{\sqrt{(-r\sin t dt)^2+(r\cos t dt)^2}}{r\sin t}= \displaystyle\int\limits_\alpha^\beta \cfrac{r dt}{r\sin t}$

Note the denominator of the final integrand can be rexpressed as $2\sin\frac{t}{2}\cos\frac{t}{2}$ and observing that

$d(\ln\tan\frac{t}{2}) = \cfrac{1}{\tan\frac{t}{2}}\ \cfrac{\sec^2\frac{t}{2}}{2}\ dt$ ,

one obtains

$\rho(\alpha,\beta)=\displaystyle\int\limits_\alpha^\beta d\left(\ln\tan\frac{t}{2}\right)=\ln\left(\cfrac{\tan\beta/2}{\tan\alpha/2}\right)$

Using trigonometric identity,

$\csc\theta-\cot\theta=\cfrac{1}{\sin\theta}-\cfrac{\cos\theta}{\sin\theta}=\cfrac{1-\cos^2\theta/2+\sin^2\theta/2}{2\sin\theta/2\cos\theta/2}=\cfrac{\sin\theta/2}{\cos\theta/2}$ ,

we can reexpress the distance (following Stahl) as

$\rho(\alpha,\beta)=\ln\left(\cfrac{\csc\beta-\cot\beta}{\csc\alpha-\cot\alpha}\right)$ .

For the case of two points on the vertical line

the distance formula (with $dx=0$) gives

$\rho(y_1,y_2)=\displaystyle\int\limits_{y_1}^{y_1} \cfrac{dy}{y}=\ln\left(\cfrac{y_2}{y_1}\right)$ .

Next, we show the distance formula given by the (claimed) geodesics indeed give the infimum distance. Consider an arbitrary curve between $P$ and $Q$ (not collinear vertically):

Using the same coordinatization $x=d+r\cos\theta,\ y=r\sin\theta$ but with $r$ depend on $\theta$, we obtain

$\cfrac{dx}{d\theta}=r'\cos\theta-r\sin\theta\ ;\quad\cfrac{dy}{d\theta}=r'\sin\theta+r\cos\theta$ .

Hence,

$dx^2+dy^2=\left(\cfrac{dx}{d\theta}\right)^2d\theta^2+\left(\cfrac{dy}{d\theta}\right)^2d\theta^2=(r'^2+r^2)d\theta^2$ .

Thus, the distance measured through arbitrary curve connecting $P$ and $Q$ obeys

$\displaystyle\int\limits_\alpha^\beta \cfrac{\sqrt{r'^2+r^2}}{r\sin\theta}\ d\theta\geq\displaystyle\int\limits_\alpha^\beta \cfrac{\sqrt{r^2}}{r\sin\theta} d\theta=\int_\alpha^\beta\csc\theta d\theta=\rho(\alpha,\beta)$ .

This shows the arc of the semicircle connecting $P$ and $Q$ gives the infimum hyperbolic distance. Next we do the same for arbitrary curves joining two vertically collinear points.

Let $x$ be a function of $y$ i.e. $x=x(y)$. Then

$\displaystyle\int\limits_{y_1}^{y_2} \cfrac{\sqrt{dx^2+dy^2}}{y}=\displaystyle\int\limits_{y_1}^{y_2} \cfrac{\sqrt{x'^2dy^2+dy^2}}{y}\geq\displaystyle\int\limits_{y_1}^{y_2} \cfrac{dy}{y}=\ln\cfrac{y_2}{y_1}$ .

Again, the vertical line joining the two points give the least distance in comparison to other curves joining them, hence the earlier-claimed geodesics.

Finally we would like to give the solution to Exercise 4 of Katok’s article where one could essentially transform any semicircle (geodesic) to the vertical line and hence making any other proofs concerning geodesics easier. Let the semicircle be centred at the origin with radius $\alpha$ and the vertical line be the $y$-axis. Both can be parametrized as $z=\alpha e^{i\theta}\ ,\ z=i\tan\theta/2$ respectively. Consider then the transformation $T(z)=-(z-\alpha)^{-1}+\beta$. Plugging in the parametrization gives

$T(z)=\cfrac{1}{\alpha(1-e^{i\theta})}+\beta=\cfrac{1+\beta\alpha(1-e^{i\theta})}{\alpha e^{i\theta/2}(e^{-i\theta/2}-e^{i\theta/2})}$ .

Putting $\beta\alpha = -1/2$ and multiplying top and bottom by $e^{-i\theta/2}$ gives

$T(z)=-\cfrac{1}{2\alpha}\ \cfrac{e^{-i\theta/2}+e^{i\theta/2}}{e^{i\theta/2}-e^{-i\theta/2}}$ .

Note that the second factor is already the reciprocal of $i\tan\theta/2$. Thus by putting $\alpha=1/2$ and composing the transformation with an inversion $S(z)=-1/z$ gives the necessary transformation i.e.

$(S\circ T)(z)=\left(\cfrac{1}{z-\frac{1}{2}} + 1\right)^{-1}$ .

References

1. S. Katok, “Fuchsian Groups, Geodesic Flows on Surfaces of Constant Negative Curvature and Symbolic Coding of Geodesics”, (http://www.math.psu.edu/katok_s/cmi.pdf)
2. R. Hayter, “The Hyperbolic Plane – A Strange New Universe” , (http://www.maths.dur.ac.uk/Ug/projects/highlights/CM3/Hayter_Hyperbolic_report.pdf)
3. S. Stahl, A Gateway to Modern Geometry: The Poincare Half-Plane, (Jones & Bartlett, 2007).
4. J. Hilgert, “Maass Cusp Forms on $\textrm{SL}(2,\mathbb{R})$“, (http://www.math.uni-paderborn.de/~hilgert/Metz05web.pdf)

## Models of Hyperbolic Plane

Resurrecting this blog. Recently I have been giving lectures on hyperbolic geometry to my students including student trainees. The lectures allow me to brush up on basics of hyperbolic geometry for upcoming conference and also to help the trainees do their mini projects. I begin with the models of hyperbolic plane.

Hyperboloid Model

Everyone knows the unit (two-) sphere whose equation is $x_1^2 + x_2^2 + x_3^2 = 1$:

If one changes the sign in one variable of the sphere-equation, $x_1^2 + x_2^2 - x_3^2 = 1$, one obtains the one-sheeted hyperboloid:

If one changes the sign of the constant on the right hand side, $x_1^2 + x_2^2 - x_3^2 = -1$, one gets the two-sheeted hyperboloid:

These disconnected pieces remind us the hyperbola curves we plotted in schools $x^2 - y^2 =\pm 1$:

One should also mention the other possibility from the two-forms of the hyperboloid earlier, the one with zero on the right hand side, giving a double cone reminiscent of the light cones in relativity.

These null surfaces are the asymptotes of the hyperboloids (very much like the asymptotes of the hyperbola).

Returning to the double-sheeted hyperboloid; let us restrict to the positive sheet namely

$L=\{(x_1,x_2,x_3)\in\mathbb{R}^3\vert x_1^2+x_2^2-x_3^2=-1,\ x_3>0\} ,$

it gives the hyperboloid model of the hyperbolic plane. The surface can be parametrised in ‘angular’ coordinates much in the same way as the sphere via

$x_1=\sinh\theta\cos\phi;\ x_2=\sinh\theta\sin\phi;\ x_3=\cosh\theta,$

which leads to $x_1^2+x_2^2-x_3^2=\sinh^2\theta-\cosh^2\theta= -1$. Note the use of hyperbolic functions. Note also the form of bilinear form one can associate to $\mathbb{R}^3$ to form the equation of the hypwerboloid i.e.

$(x,y)_{2,1} = x_1y_1 + x_2y_2 - x_3y_3\$

Using this bilinear form, one could actually form the metric on $L$ by forming the product $(dx,dx)_{2,1}$ to give

$ds^2 = dx_1^2+dx_2^2-dx_3^2\ .$

Beltrami-Klein Model

In the hyperboloid model $L$ above, it is tangent to the plane $K$ of $x_3=1$. Next we project points on $L$ to this plane by drawing a line to the origin (see Fig below).

The point $l=(x_1,x_2,x_3)\in L$ is projected to the point $k=(\eta_1,\eta_2)\in K$. By using similar triangles, $\eta_i=\eta_i/1=x_i/x_3$ for $i=1,2$. Note that

$\eta_1^2+\eta_2^2=\cfrac{x_1^2+x_2^2}{x_3^2}=\cfrac{x_3^2-1}{x_3^2}=1-\frac{1}{x_3^2}<1$.

Thus under this project the hyperboloid $L$ is mapped to a unit disk $K$, which is the Beltrami-Klein disk model of the hyperbolic plane. By using the inverse coordinate transformation

$x_i=\cfrac{\eta_i}{\sqrt{1-\eta_1^2-\eta_2^2}}\ ,\ (i=1,2);\quad x_3=\cfrac{1}{\sqrt{1-\eta_1^2-\eta_2^2}}$,

one can get the hyperboloid differentials in terms of these new coordinates as

$dx_i=\cfrac{(1-\eta_1^2-\eta_2^2)d\eta_i+\eta_i(\eta_1 d\eta_1+\eta_2 d\eta_2)}{(1-\eta_1^2-\eta_2^2)^{3/2}}$

for $i=1,2\$ and

$dx_3=\cfrac{(\eta_1 d\eta_1+\eta_2 d\eta_2)}{(1-\eta_1^2-\eta_2^2)}$.

Hence the metric for the B-K disk to be

$ds^2=\cfrac{d\eta_1^2+d\eta_2^2}{1-\eta_1^2-\eta_2^2} + \cfrac{(\eta_1d\eta_1+\eta_2d\eta_2)^2}{(1-\eta_1^2-\eta_2^2)^2}$.

Hemispherical Model

Another interesting model is to project the points of hyperboloid further to a hemisphere $J$ (would aid us later to get other more convenient models).

The point $l$ is now projected down to $j=(\eta_1,\eta_2,\eta_3)$ from $k$ of B-K model where the additional coordinate is given by $\eta_3=1/x_3,\ x_3>0$. It is easy to show that these coordinates now form the equation of a (hemi-)sphere

$\eta_1^2+\eta_2^2+\eta_3^2=1$.

Hence, the hemisphere model of the hyperbolic plane. Note that this (hemi-)sphere has a different metric inherited from the hyperboloid i.e.

$ds^2=\cfrac{d\eta_1^2+d\eta_2^2+d\eta_3^2}{\eta_3^2}$.

Poincare Disk Model

Using the hemisphere model, we can now make a stereographic projection from $J$ to the plane $\eta_3=0$. The point $j=(\eta_1,\eta_2,\eta_3)$ gets mapped to $i=(\xi_1,\xi_2)$ on the Poincare disk $I$.

The relation between these coordinates are given by the similar triangle relations

$\cfrac{\xi_1}{\eta_1}=\cfrac{\xi_2}{\eta_2}=\cfrac{1}{1-\eta_3}$.

One can work backwards (easier) to show that the Poincare disk metric

$ds^2= \cfrac{4(d\xi_1^2+d\xi_2^2)}{(1-\xi_1^2-\xi_2^2)}$

is equivalent to the metric of the hemisphere model earlier.

Upper Half-Plane Model

Now in the Poincare disk, we use the stereographic projection from $(0,0,-1)$ to $\eta_3=0$ plane. We could use a different projection namely from $(0,-1,0)$ on the hemisphere $J$ to the plane $\eta_2=0$. The point $j= (\eta_1,\eta_2,\eta_3)$ gets mapped to $h=(x,y)$ on the plane mentioned.

The relation again between these coordinates are given by the similar triangles:

$\cfrac{x}{\eta_1}=\cfrac{y}{\eta_3}=\cfrac{1}{1-\eta_2}$.

Unlike the earlier stereographic projection that squashes the hemisphere into a bounded (Poincare) disk, the present projection gives an unbounded plane due to the point $(0,-1,0)$ gets mapped to infinity. Note also that $\eta_3>0$ implies that $y>0$. The metric can also be worked (as in the previous case) to

$ds^2= \cfrac{dx^2+dy^2}{y^2}$.

This upper half plane can be realised as

$H=\{z\in\mathbb{C}\vert\textrm{Im}(z)>0\}$.

This is probably one of the most convenient model to work with. It can be realised as a homogeneous space. Consider the action of $\textrm{SL}(2,\mathbb{R})$ on $H$ given by the fractional linear transformation

$\begin{pmatrix} a&b\\c&d \end{pmatrix} \cdot z \mapsto \cfrac{az+b}{cz+d}\equiv z'$

where $a,b,c,d\in\mathbb{R};\ ad-bc=1$. Note the action of $\begin{pmatrix} -a&-b\\-c&-d \end{pmatrix}$ gives the same effect. Thus the effective group is really $\textrm{PSL}(2,\mathbb{R}) = \textrm{SL}(2,\mathbb{R})/\pm I$. The group action could also be conveniently put into matrix multiplication form in the following way:

$\begin{pmatrix} a&b\\c&d \end{pmatrix}\cdot \begin{pmatrix} z\\1 \end{pmatrix} = \begin{pmatrix} az+b\\cz+d \end{pmatrix} \equiv \begin{pmatrix} \frac{az+b}{cz+d}\\1 \end{pmatrix}$.

It would seem like that each point on $H$ can be made in correspondence with each element of $\textrm{PSL}(2,\mathbb{R})$ but one ought to check if any of the group elements keep the point fixed. Consider for example the subgroup $\textrm{SO}(2)\subset\textrm{SL}(2,\mathbb{R})$. Elements $\begin{pmatrix} \cos\theta&-\sin\theta\\ \sin\theta&\cos\theta \end{pmatrix}\in\textrm{SO}(2)$ keeps $z=i$ fixed:

$\begin{pmatrix} \cos\theta&-\sin\theta\\ \sin\theta&\cos\theta \end{pmatrix}\ \begin{pmatrix} i\\1 \end{pmatrix}= \begin{pmatrix} i(i\sin\theta+\cos\theta)\\ i\sin\theta+\cos\theta \end{pmatrix}\equiv \begin{pmatrix} i\\1 \end{pmatrix}$.

Thus in general for each point there will be effectively an (conjugated) $\textrm{SO}(2)$ that keeps the point fixed. Thus the upper half plane can be realised as the quotient space $\textrm{PSL}(2,\mathbb{R})/\textrm{PSO}(2)$.

In the next few forthcoming posts, we will investigate this model of the hyperbolic plane further

References

1. Pages from http://en.wikipedia.org/wiki/Main_Page
2. S. Katok, “Fuchsian Groups, Geodesic Flows on Surfaces of Constant Negative Curvature and Symbolic Coding of Geodesics”, (http://www.math.psu.edu/katok_s/cmi.pdf)
3. J.W. Cannon, W.J. Floyd, R. Kenyon & W.R. Parry, “Hyperbolic Geometry” (http://www.math.brown.edu/~rkenyon/papers/cannon.pdf)
4. R. Hayter, “The Hyperbolic Plane – A Strange New Universe” (http://www.maths.dur.ac.uk/Ug/projects/highlights/CM3/Hayter_Hyperbolic_report.pdf)
5. J. Hilgert, “Maass Cusp Forms on $\textrm{SL}(2,\mathbb{R})$“, (http://www.math.uni-paderborn.de/~hilgert/Metz05web.pdf)

## Making Spaces

This is a set of notes from some talk way back in February 2011 and I decided to type it out and it continues naturally from the last post on set theory.

The question posed is what is a space? More often than not, physicists tend to think that spaces are ‘places’ where objects can be contained in very much like the space(-time) we live in. But these ideas have further properties with them such as breaking them into subregions, joining regions into bigger ones, distinguishability of points and regions, all of which are captured in the mathematicians’ notion of topology.

A topology on a set $X$ is a collection $\mathcal{T}$ of subsets of $X$ called open sets satisfying

1. $X$ and $\emptyset$ are elements in $\mathcal{T}$
2. Closure under finite intersections
If $U_1, U_2, \dotsc , U_n \in \mathcal{T}$, then $U_1\cap U_2 \cap \dotsc U_n \in \mathcal{T}$.
3. Closure under arbitrary unions
If $\{ U_\alpha \}_{\alpha\in A}$ is any collection of elements in $\mathcal{T}$ (labelled by index set $A$, then $\cup_{\alpha\in A} U_\alpha \in \mathcal{T}$.

The pair $(X, \mathcal{T})$ is called a topological space. Often the case, physicists tend to refer to $X$ as being the space itself with the understanding some standard choice of topology has been made.

Before elaborating on further ideas of topological space, we need to make known some intuitive ideas of what is an open set. First, we take the set $\mathbb{R}$ with an ordered relation $<$. Intervals of $\mathbb{R}$ form needed subsets. For example,

$(a, b) = \{ x\in\mathbb{R} \vert a < x < b\}$ is an open interval.

$[ a, b] = \{ x\in \mathbb{R} \vert a \leq x \leq b \}$ is a closed interval.

Note that $(a, b)$ form the open sets while $[a, b]$ form the closed sets. The open intervals in fact form the standard topology on $\mathbb{R}$.

Closed sets are different creatures but not unrelated.  A set $A$ if topological space $X$ is closed if set $X-A$ is open i.e. $(X-A)\in \mathcal{T}$. For example, $[a, \infty)$ is closed since $(-\infty, a)$ is open. In fact there is some form of duality relationship between the two. One can even define a topological space using closed sets.

Let $X$ be a topological space. Then

1. Arbirary intersections of closed sets in $X$ are closed.
2. Finite union of closed sets in $X$ are closed.

Note also the dual behaviour of the set operations of union and intersection. It is important to note that $X, \emptyset$ are both open and closed. Taking the earlier as definition then the empty set is closed since its complement $X$ is open;  and $X$ is also closed since its complement $\emptyset$ is open. A set that is both closed and open is called clopen.

To prove the first point, consider $\{ A_\alpha \}_{\alpha\in J}$ of closed sets. By deMorgan’s law, we have

$X - \cap_{\alpha\in J} A_\alpha = \cup_{\alpha\in J} (X - A_\alpha)$.

But $(X-A_\alpha)$ is open and arbitrary union of open sets are open. Hence $(X-\cap_{\alpha\in J} A_\alpha)$ is open, which then implies $\cap_{\alpha\in J} A_\alpha$ is closed.

For the second point, consider closed sets $A_i$ for $i=1,\dotsc , n$. By de Morgan’s law, we have

$X - \cup_{i=1}^n A_i = \cap_{i=1}^n (X-A_i)$

Since $(X-A_i)$ is open and finite intersections of open sets are open, then $(X-\cap_{i=1}^n A_i)$ is open or $\cap_{i=1}^n A_i$ is closed. QED.

A set can also be neither open nor closed. Consider $\mathbb{R}$ with the standard topology. The intervals $(-\infty, a)$ and $(a,\infty)$ are both open but $[ a,b)$ is not open and it is also not closed. This can be understood in the following way; its complement being

$\mathbb{R} - [a,b) = (-\infty, a) \cup [b,\infty)$

is the union of an open set and a closed set. Thus the said property of $[a,b)$.

It is important to note that the open property of a set is not really universal. We can see this by setting up a counterexample. Consider the set $Y = [0,1]\cup [2,3]\subset \mathbb{R}$ as a topological space with the standard topology. Now $[0,1]$ is open in $Y$ since $Y - [0,1] = [2,3]$ is closed. But then $[0,1]$ is closed in $\mathbb{R}$; hence the open property does not carry over to the container space.

Having said much about the idea of open sets that underlies the definition of topological space, we can now see the reason why in defining $(X,\mathcal{T})$, we only allow for finite intersections of open sets. If this limitation is lifted, then one can consider for example, the intersection of all intervals $(-\frac{1}{n}, n)$ of $\mathbb{R}$. The result would give the set $\{ 0 \}$, which is not open (hence failing the condition).

Let’s try to go beyond one-dimensional $\mathbb{R}$ and discuss the analogue of open intervals for open sets. One problem is the ordering property of $\mathbb{R}$ is no longer there. Thus one has to invoke other structures for the purpose of introducing open sets. Consider $\mathbb{R}^2$ as the metric space $(\mathbb{R}^2, d)$ with standard (Euclidean) metric $d$. Then

$D = \{ y\in\mathbb{R}^2 \vert d(x,y) < R\}$

is an open disk centred at $x\in \mathbb{R}^2$ and this serves as the open sets for $\mathbb{R}^2$. Taking its closure,

$\overline{D} = \{ y\in\mathbb{R}^2 \vert d(x,y) \le R\}$

is a closed disk and this serves as the closed sets. It is now easy to see how this can be generalised to an open (closed) ball in $(\mathbb{R}^n, d)$ and in higher-dimensional metric spaces. In fact, the generalisation does not stop there. One can build spaces much different from the usual familiar notion of spaces such as space of functions (important in functional analysis), spaces of lines, spaces of solutions etc. For a good read of examples of topological spaces, try this. A route to further generalisation in this case is to move from open sets to related ideas of neighbourhoods of points and to ideas of basis and filiters over which issues like convergence can be discussed. This is beyond the scope of the present post.

Let us however get more elementary for the moment and consider sets of discrete elements to demonstrate further ideas of topology. Consider the following examples over the set $X=\{1, 2, 3\}$.

Let $\mathcal{T}_1 = \{ \emptyset, \{ 1. 2 \}, \{ 2, 3 \}, X\}$. Checking on its topology property e.g.

$\{ 1, 2\} \cup \{ 2, 3\} = X \in \mathcal{T}_1$

which seems to be fine but the following property

$\{ 1, 2 \} \cap \{ 2, 3\} = \{ 2\} \notin \mathcal{T}_1$

fails. Thus $\mathcal{T}_1$ is not a topology on $X$.

If we let $\mathcal{T}_2 = \{ \emptyset, \{ 1\}, \{ 2,3\}, X \}$, then a check on its topological properties shows that it is a valid topology on $X$ e.g.

$\{ 1\} \cap \{ 2, 3\} = \emptyset \in \mathcal{T}_2$

$\{ 1\} \cup \{ 2, 3\} = X \in \mathcal{T}_2$

Next, we can consider $\mathcal{T}_3 = \{ \textrm{all subsets of } X\}$. It can be easily checked that $\mathcal{T}_3$ forms a topology on $X$. The topology is called a discrete topology on $X$ and $(X, \mathcal{T}_3)$ is called a discrete space.

We could also form the topology $\mathcal{T}_4 = \{ \emptyset , X \}$ and this is said to be the trivial topology on $X$. The different topologies can be depicted in the following picture.

The above example shows that one can define different topologies on a set/space. Note also that $\mathcal{T}_4 \subset \mathcal{T}_2 \subset \mathcal{T}_3$ and we say this as $\mathcal{T}_4$ is coarser than $\mathcal{T}_2$ and $\mathcal{T}_3$ is finer than $\mathcal{T}_2$. The finer topologies allow us to distinguish more points of the space (later we will discuss separation axioms).

Now, two different definition of topologies may not necessarily give different topological  spaces. Consider back the metric space $(\mathbb{R}^2, d_E)$ with Euclidean metric  $d_E (x,y) = \left(\sum_{i=1}^2 (x_i - y_i)^2\right)^{1/2}$ whose open set is the open disk. We can equip the space with a different metric, the taxicab metric, $d_T (x,y) =\sum_{i=1}^2 \vert x_i - y_i \vert$ whose open set will be an open square. One can inscribe the square in the disk indicating the $d_T$-induced topology finer that the $d_E$-induced topology. But so can the disk be inscribed in the square indicating the reverse. Herce in this case, they are equivalent as topological spaces.

The final point for this post is how can we separate points in space. Often mentioned in theoretical physics literature, when defining a manifold, is the Hausdorff property. A topological space $X$ is Hausdorff if given any pair of distinct points $x_1, x_2 \in X$, there exists neighbourhoods (open sets cointaining respective points) $U_1$ of $x_1$ and $U_2$ of $x_2$ such that $U_1 \cap U_2 = \emptyset$. Note the utility of open sets separating points.

There can be different degrees of variability on how we separate points. We list the different axioms of separability for reader’s reference.

$T_0$ axiom: If $x_1, x_2 \in X$, there exists open set $O \in\mathcal{T}$  such that
$(x_1\in O \wedge x_2\notin O) \vee (x_2\in O\wedge x_1\notin O)$.
A $T_0$ space is called a Kolmogorov space.

$T_1$ axiom: If $x_1, x_2 \in X$, there exist open sets $O_1, O_2 \in\mathcal{T}$ such that
$(x_1\in O_1 \wedge x_2\in O_2) \wedge (x_2\notin O_1 \wedge x_1\notin O_2)$.
A $T_1$ space is called a Frechet space.

$T_2$ axiom: If $x_1, x_2 \in X$, there exist open sets $O_1, O_2 \in\mathcal{T}$ such that
$(x_1\in O_1 \wedge x_2 \in O_2) \wedge (O_1\cap O_2 = \emptyset)$.
This is the case of the Hausdorff space.

$T_3$ axiom: If $A$ is a closed set with $x_2\notin A$, there exist open sets $O_A, O_2$ such that
$(A\subset O_A \wedge x_2 \in O_2) \wedge (O_A \cap O_2 = \emptyset)$.

$T_4$ axiom: If $A, B$ are disjoint closed sets in $X$, there exist open sets $O_A, O_B$ such that
$(A\subset O_A \wedge B\subset O_B) \wedge (O_A\cap O_B = \emptyset)$.

$T_5$ axiom: If $A, B$ are separated sets (i.e. $\bar{A} \cap B = \emptyset = A \cap \bar{B}$) in $X$, there exist open sets $O_A, O_B$ such that
$(A\subset O_A \wedge B\subset O_B) \wedge (O_A\cap O_B = \emptyset)$.

Combining conditions $T_1 \wedge T_2$ gives what is known as regular space and combining $T_1 \wedge T_4$ gives a normal space. We also have the $T_{2\frac{1}{2}}$ space where the $T_2$ axiom is supplemented by the condition $\bar{O}_1 \cap \bar{O}_2 = \emptyset$, which is called a completely Hausdorff space. Most of these differences are however ignored by theoretical physicists.

References

List of books referred to for this post.

## What They Don’t Teach You About Sets 2

This is the second installment of notes of my lecture in LuFTER 1/2011. In the last post, we are limited to relationships on a given set. With so limited tools, we will not be able to do much apart from say, solving taxonomic problems. Here, we will now introduce more structures, namely operations on sets.

Set Operations

The two basic operations of set theory are the well-known set union and set intersection.

The union $X\cup Y$ of sets $X$ and $Y$ is the set of objects whose members are either members of $X$ or members of $Y$ i.e.

$X\cup Y=\{x:(x\in X)\lor(x\in Y)\}$ .

The set union obeys commutativity

$X\cup Y=Y\cup X$ ,

and associativity

$X\cup(Y\cup Z)=(X\cup Y)\cup Z$ .

The intersection $X\cap Y$ of sets $X$ and $Y$ is the set of objects whose members are members of both $X$ and $Y$ i.e.

$X\cap Y=\{x: (x\in X)\land(x\in Y)$ .

Similarly the set intersection also obeys commutativity and associativity:

$X\cap Y=Y\cap X$ ;

$X\cap(Y\cap Z)=(X\cap Y)\cap Z$ .

Exercise 1: Prove the commutativity and associativity laws for the set union and intersection.

At this juncture, one should probably have noticed the correspondence between set union and intersection with the logical or and logical and respectively. Later, we will see that these set operations do indeed give the algebra of Boolean logic. (Question: Which comes first?) It is also good to highlight one can extend both union and intersection to a family of sets $X_i$ where $i\in I$ for some labelling set $I$; namely $\bigcup_{i\in I} X_i$ and $\bigcap_{i\in I} X_i$. You will probably see them in defining topological space.

One could go further to define the idea of set-theoretic difference $X-Y$ (perhaps as opposed to the union) as

$X-Y=\{x:(x\in X)\land(x\notin Y)$ .

Sometimes, it is also denoted as $X\backslash Y$. An example is the set difference $\mathbb{R}-\mathbb{Q}$ between the set of real numbers $\mathbb{R}$ and the set of rational numbers $\mathbb{Q}$; the resultant set is the set $\mathbb{J}$ of irrational numbers.

Exercise 2: Prove $X-Y=X-(X\cap Y)$.

Note that if $X\cap Y=\emptyset$, then $X$ and $Y$ is said to be disjoint. Related to the set difference operation is the symmetric difference i.e.

$X\Delta Y=(X-Y)\cup(Y-X)$ ,

and this has “better” properties:

• $X\Delta Y=Y\Delta X$;
• $X\Delta Y=(X\cup Y)-(Y\cap X)$;
• $(X\Delta Y)\Delta Z=X\Delta(Y\Delta Z)$.

Often one build sets from one large set (the universe of discourse) and this large set, we called universal set $U$. Within such set then, we can now define an operation called complement $X^c$ of the set $X\subset U$:

$X^c=\{x:x\in U\land x\notin X\}$ .

This complement operation obeys the following:

• $(X^c)^c=X$;
• $\emptyset^c=U$;
• $U^c=\emptyset$;
• $X\cup X^c=U$;
• $X\cap X^c=\emptyset$.

The complement operation also obeys the deMorgan’s laws:

• $(X\cup Y)^c=X^c\cap Y^c$;
• $(X\cap Y)^c=X^c\cup Y^c$.

One can see that the complement really acts like a NOT gate.

Exercise 3: Prove the properties of the complement operation including deMorgan’s laws.

To complete the logic operations analogy, one adds further the distributivity law whenever one mixes both the union and intersection operations together:

• $X\cup(Y\cap Z)=(X\cup Y)\cap(X\cup Z)$;
• $X\cap(Y\cup Z)=(X\cap Y)\cup(X\cap Z)$.

Exercise 4: Prove the distributive laws.

Putting together all the operations together with the sets $X,Y,Z$ in $U$, they form what is known as the Boolean algebra.

We end this part by giving an interesting example of constructing the “natural numbers” using set operations. We begin with the empty set $\emptyset$, which we conveniently called it $0$. Next, we define what we called as the successor set $S(n)=n\cup\{n\}$ giving the following sequence (with more relabelling):

• $0:=\emptyset$;
• $1:=S(0)=0\cup\{0\}=\emptyset\cup\{0\}=\{0\}$;
• $2:=S(1)=1\cup\{1\}=\{0\}\cup\{1\}=\{0,1\}$;
• $3:=S(2)=2\cup\{2\}=\{0,1\}\cup\{2\}=\{0,1,2\}$;
• $\qquad\vdots$

The collection $\{0,1,2,3,\cdots\}$ forms the set $\omega$ of natural numbers, which can be shown to obey the Peano postulates:

• $0\in\omega$;
• $\forall n\in\omega\Rightarrow S(n)\in\omega$;
• $\forall n,m\in\omega\ (n\neq m)\Rightarrow(S(n)\neq S(m))$;
• $\forall X\subset\omega,\ ((0\in X)\land(\forall n\in X(S(n)\in X)))\Rightarrow X=\omega$.

The set $\omega$ can then be identified with the natural numbers $\mathbb{N}=\{0,1,2,\cdots\}$. One could easily see that for $m,n\in\omega$, then $m\in n\Rightarrow m\subset n$ implying the orderedness of numbers, a property that we will touch upon later. In fact we could do more by defining addition and multiplication using set-theoretic relations and operations.

It is often remarked that this example is like creating something (numbers) out of nothing (the empty set). In some sense, it is but there is no need to philosophize it too much (and in no way it is like the creation operation!). It is simply an abstraction of the counting operation. An analogy can be made with by first peering into an empty box and later put the empty box in another box, and the latter box in one other box ad infinitum, very much like the Russian doll. If there is anything to philosophize on, is that numbers can be thought of as an abstract mental construct that we associate with counting – a fact that we often forget.

References

These are additional references since the first part:

## What They Don’t Teach You About Sets 1

Resurrected this blog (again) for posting my postgraduate lectures/talks here. Here are the notes from LuFTER 1/2011 February 9, 2011. The Lunchtime Foundational Theory Expositions and Ruminations will consists of mainly lectures to my postgraduate students and occasionally research seminars, proposals etc.

The present ongoing lectures are meant to introduce mathematical structures in theoretical physics with perhaps Nirmala Prakash’s book as a guide of topics to be covered (but not necessarily adhering to it). The book itself is skewed towards topics for string theory but we will also cover other topics. The first topic will be on set theory. The mischievous title of the lecture is taking cue from Devlin’s book entitled “The Joy of Sets”, which is referenced.

Relationships and Sets

In most (conventional) physics, often use very rich mathematical structures (e.g. differential geometry) from the outset, which assume many things. Advances in foundational theories (e.g. quantum gravity) tend to relook at the usage basis of these structures and either generalize them or opt for primitive structures. So, what would be considered the most primitive structure? Set theory seems to fit the role with its common usage as a mathematical language. However, usually physicists are only exposed to very elementary ideas of set theory and perhaps not see its full power often only found in more advanced mathematics course. We hope to remedy this a little. In fact the deeper usage of set theory is really the following:

• understanding the infinite
• foundational subject matter of mathematics
• common mode of reasoning

So what is a set? Its fundamental idea is simply the ability to regard a collection of objects as a single entity (the set). This sounds circular. In fact, in set theory, the undefinables are really the notion of a set and the relation “is an element of”. We introduce notation:

• $x \in X$” which means “$x$ is an element of $X$“;
• $x \notin X$” which means “$x$ is not an element of $X$“.

In forming sets, one can either

• enumerate the elements or members of the set e.g. $X = \{ x_1 , x_2 , \cdots , x_n , \cdots \}$; or
• describe by using some property $P$ e.g. $X = \{ x : P(x) \}$, which is the set of all $x$ for which $P(x)$ holds. Example: $\mathbb{C} = \{ z : z \textrm{ is a complex number} \}$.

Using normal sentences to describe a set may not be best. Better, use logical statements. For this, we introduce the logical notations:

• $\Rightarrow$ means “implies”;
• $\Longleftrightarrow$ means “if and only if”;
• $\neg$ means “not”;
• $\wedge$ means “and”;
• $\vee$ means “or”;
• $\forall$ means “for all”;
• $\exists$ means “there exists”.

With these, one can start building logical statements for the set building. Some examples logical statement are given below (which also shows that some logical operations can be “derived” from others.

Example 1: $P \Longleftrightarrow Q$ is the same as $(P\Rightarrow Q) \land (Q\Rightarrow P)$
One can build the truth table to show this is true.

$\begin{array}{c|c|c} P\Rightarrow Q & Q\Rightarrow P & (P\Rightarrow Q)\land(q\Rightarrow Q)\\ \hline \textrm{False} & \textrm{False} & \textrm{False}\\ \textrm{False} & \textrm{True} & \textrm{False}\\ \textrm{True} & \textrm{False} & \textrm{False}\\ \textrm{True} & \textrm{True} & \textrm{True}\end{array}$

Note that we could have build the truth table out of atomic statements instead of compound ones but we leave this for the reader to elaborate.

Exercise 1: Show that $P\Rightarrow Q$ is the same as $(\neg P)\vee Q$.

Example 2: $P\vee Q$ is the same as $\neg((\neg P)\land(\neg Q))$.
Now we build the truth table from the atomic statements for illustration.

$\begin{array}{c|c|c|c|c|c} P & Q & \neg P & \neg Q & (\neg P)\land(\neg Q) & \neg((\neg P)\land(\neg Q))\\ \hline \textrm{False} & \textrm{False} & \textrm{True} & \textrm{True} & \textrm{True} & \textrm{False}\\ \textrm{False} & \textrm{True} & \textrm{True} & \textrm{False} & \textrm{False} & \textrm{True}\\ \textrm{True} & \textrm{False} & \textrm{False} & \textrm{True} & \textrm{False} & \textrm{True}\\ \textrm{True} & \textrm{True} & \textrm{False} & \textrm{False} & \textrm{False} & \textrm{True}\end{array}$

This example is illustrative of the fact that one does not need all the logical operations; here, the “or” operation has been replaced by a combination of a “not” and an “and”. It is in fact well known that the “nand” gate (combining “not” and “and”) is a universal gate for classical computations.

In handling or consructing sets abstractly, it is important to ponder on Quine’s dictum “No entity without identity”. How would one know that two so-called abstract entities (sets) are not one and the same? Here, we state the following axiom of extensionality telling us when two sets $X$ and $Y$ are the same:

$X=Y \iff \forall x (x\in X) \Longleftrightarrow (x\in Y)$ .

Thus, for example $\{a,b,c\} = \{c,a,b\}$. In this regard, there is one important set that we need to construct, namely the empty set $\emptyset$ or $\{\}$. It can be defined as

$\emptyset = \{ x: x \neq x \}$ .

Note from this definition, in principle we could have started with $x$ coming from different sets, say $X$ giving $\emptyset_X$ and $\chi$ giving $\emptyset_\chi$. But by virtue of axiom of extensionality, the empty set is unique, being the set with no elements (Exercise 2: Prove this).

Exercise 3: Using results of exercise 1, prove the statement $x\in\emptyset \Rightarrow P(x)$ is true for all $x$.

An easy way to define more sets is to consider another set relation, namely subsets. We define $Y$ is a subset of $X$, written as $Y\subseteq X$, if and only if every element of $Y$ is an element of $X$ i.e.

$Y\subseteq X\Longleftrightarrow\forall x((x\in Y)\Rightarrow(x\in X))$ .

Note that in this case $X$ is also called superset of $Y$ i.e. $X\supseteq Y$. In both these relations, it is possible that $X=Y$. But if you would like to consider otherwise i.e. considering $Y$ is a proper subset of $X$, then we write $Y\subset X$ where

$Y\subset X\Longleftrightarrow(Y\subseteq X)\land(Y\neq X)$ .

Another important set concept is the idea of a power set $\mathcal{P}(X)$ of the set $X$, which is the set of all subsets of $X$ i.e.

$\mathcal{P}(X) = \{ Y :Y\subseteq X\}$ .

We will illustrate the idea of power sets in a minute but before that one introduces another set relation namely the cardinality. Cardinality is a measure of the “size’ of the set particularly by looking into the “number” of its elements. We denote the cardinality of a set $X$ by $\sharp(X)$ or $\textrm{card}(X)$.

Example 3: Consider the set $X=\{\Diamond\}$. Its cardinality is $\sharp(X)=1$. The power set of $X$ is $\mathcal{P}(X)=\{\emptyset,\{\Diamond\}\}$ and hence $\sharp\mathcal{P}(X)=2$. One could proceed further to find the power set of $\mathcal{P}(X)$ itself:

$\mathcal{P}(\mathcal{P}(X))=\{\emptyset,\{\emptyset\},\{\Diamond\},\{\emptyset,\{\Diamond\}\}\}$ ;

with $\sharp\mathcal{P}(\mathcal{P}(X))=4$. In fact, one can iterate this $n$ times and find $\sharp\mathcal{P}^{(n)}(X) = 2^n$. It is interesting to note that the empty set played a role in increasing the cardinality of the nested power sets. We will later show that one can do better than this to build up what we know as numbers.

To end this part, I add a cautionary note that not all collections of objects can form a set. Consider the Russell set:

$R=\{x:x\notin x\}$ .

Let’s ask: does $R$ satisfies the property given? If yes, then $R\notin R$ but then by the set definition $R\in R$ – giving a contradiction. Suppose the answer is no then, which means $R\in R$. However the set definition implies $R\notin R$ – again, a contradiction. This is essentially known as the Russell’s paradox. It’s resolution? Perhaps, it is too much to impose $R$ to be a set, but $R$ is said to be a class (will not elaborate here). Essentially the idea is to differentiate the use of a symbol and the meaning of the symbol. Alternatively, one could axiomatize set theory using Zermelo-Fraenkel axioms, which is beyond the scope of the lecture.

References

I list here some of my reading materials without attempting to show where I have used them. Go read them, if you have interest.