# Algebraic Geometry, or Why I’m a moron

There are two main things to note before I hit the main components of this rant tryst exposition. Number one:

In my department, Algebraic Geometry is a big deal. We have two (count it: one, two) algebraists whose expertise in the subject is second-to-none, and we have another cluster who – despite being cross denominational in their research – are truly masters in the field.

In my department, Algebraic Geometry is a big deal.

It’s unsurprising, too, I guess: Since Grothendieck revamped the field in the 50s and 60s, its usefulness has been realized to be extremely wide-spread and, as such, people really really care about it.

In my department, Algebraic Geometry is a big deal. That’s the first thing to keep in mind.

# Hodge Dual part deux

You may recall that I deemed yesterday Differential Geometry Sunday and posted a small expository thing on the Hodge Star/Dual operator. Apparently in my cloudy haze of mathematical mediocrity, I concluded my post without having touched on the derivations I actually intended to touch on.

Sometimes I feel like I need a vacation from my vacation.

In any case, I’m going to take a stab at saying some of the things I’d meant to say yesterday, but in order to ensure we’re all on the same page, I’m going to recall what exactly the Hodge Star/Dual operator is. Then, after the break, I’m going to show some of the cool derivations that come about because of it.

Let $M$ be a manifold of dimension $\dim(M)=n$ and let $\omega=\sum_I f_I dx_I$ be a $k$-form on $M$ for $k\leq n$. Here, $I$ denotes a multi-index which – without loss of generality – can be assumed to be increasing. Then the hodge star operator is the operator which takes $\omega$ to the $(n-k)$-form $*\omega =\sum_I f_I(*dx_I)$ where $*dx_I=\varepsilon_I dx_{I^C}$, where $I^C$ is the multi-index consisting of all numbers $1,\ldots,n$ not in $I$, and where $\varepsilon_I=\pm 1$ denotes the sign of $dx_I dx_{I^C}$.

So what we said is that for the most commonly-recognized example $\mathbb{R}^3$ with orthogonal 1-forms $dx,dy,dz$ and the usual metric $ds^2=dx^2+dy^2+dz^2$, the Hodge Star operator sends $1$ to $dx\wedge dy\wedge dz$ and vice versa, it sends $dx,dy,dz$ to $dy\wedge dz, dz\wedge dx,dx\wedge dy$ respectively, and it sends $(dx\wedge dy)\mapsto dz$, $(dy\wedge dz)\mapsto dx$, and $(dx\wedge dz)\mapsto -dy$. But the question then remains: Why does anybody care?

# Differential Geometry Update, or Things I did not know….

I’ve had a moderate amount of exposure to the study of differential forms in the context of pure differential geometry, as well as in the background of studies in hypercomplex analysis, abstract algebra, etc. Despite all that, I apparently never learned the actual definition of a differential form.

Recall that the covector space $T_p^*(M)$ is the dual vector space for the vector space $T_p(M)$ of (tangent) vectors to $M$ at $p$. Elements of are called covectors. An assignment of a covector at each point of a differentiable manifold is called a differential form of degree 1.

Seriously, guys: Who knew?

# Catching Up

So I managed to go nine months without ever looking at this thing. That’s unfortunate. It is, however, an indication of how the past two semesters have been for me, time-wise.

Without moving backwards, I’ll leave this for an update:

• My GPA is pretty not-terrible.
• I managed to exempt all of my qualifying exams by scoring A- or higher in the classes needed.
• I’ve gotten to know some professors who seem not to hate me.
• I’ve become a better mathematician.

All of those are important, of course, but the last one is the one that matters most.

When I started this thing, I’d meant it to be a place for sharing lots of general math stuff. Now that I (hopefully) have more time, I’m expecting more sharing to happen. Just to get things off on the right foot, here’s part of a little something I worked on last semester.

Here, we’re considering functions $f:\mathbb{H}\to\mathbb{H}$ which are quaternion-valued functions of a quaternion variable. The function $f$ is said to be (Feuter) Regular at $q\in\mathbb{H}$ provided $\overline{\partial}_\ell f(q)=0$, where

$\overline{\partial}_\ell=\dfrac{\partial}{\partial t}+i\dfrac{\partial}{\partial x}+j\dfrac{\partial}{\partial y}+k\dfrac{\partial}{\partial z}$

The Cauchy-Fueter Integral Formula
If $f$ is regular at every point of the positively-oriented parallelepiped $K$ and $q_0$ is a point in the interior $K^\circ$ of $K$, then

$f(q_0)=\dfrac{1}{2\pi^2}\displaystyle\int_{\partial K} \dfrac{(q-q_0)^{-1}} {|q-q_0|^2} \,Dq\,f(q).$

Proof. The proof given largely follows the outline given by Sudbury. For an equivalent statement translated from the language of differential forms into the framework of vector fields and their integrals, see the proof on page 12 of Deavours.

First, let $K$ be a 4-parallelepiped in $\mathbb{H}$ and let $q_0$ be an element of its interior $K^\circ$. Define the function $g$ so that

$g(q)=-\partial_r\left(\dfrac{1}{|q-q_0|^2}\right)=\dfrac{(q-q_0)^{-1}}{|q-q_0|^2},$

where

$\partial_r=\dfrac{\partial}{\partial t}-\dfrac{\partial}{\partial x}i-\dfrac{\partial}{\partial y}j-\dfrac{\partial}{\partial z}k.$

Clearly, then, $g$ is real differentiable at every point $q\neq q_0$ in $K$. Moreover, a simple computation verifies that $\overline{\partial}_r g=0$ except at $q=q_0$. From other results (see Sudbury), it follows that $d(g\,Dq\,f)=0$ for all $q_0\neq q\in K^\circ$ by regularity of $f$.

Next, we use the dissection method from Goursat’s theorem. In particular, consider replacing $K$ by a smaller parallelepiped $\hat{K}$ for which $q_0\in\hat{K}^\circ\subset K$. Because $f$ has a removable singularity at $q=q_0$, it follows that $f$ is continuous at $q_0$, whereby it follows that choosing $\hat{K}$ small enough allows approximation of $f(q)$ by $f(q_0)$. Finally, let $S$ be a 3-sphere with center $q_0$ and Euclidean volume element $dS$ on which

$Dq=\dfrac{(q-q_0)}{|q-q_0|}dS.$

Such substitution is possible by way of a change of variables in the definition of $Dq$. This change of variables results in the following:

$\begin{array}{rcl} \displaystyle\int_{\partial K}\dfrac{(q-q_0)^{-1}}{|q-q_0|^2}Dq\,f(q) & = & \displaystyle\int_{\partial K}g\,Dq\,f(q) \\[1.5em] & = & \displaystyle\int_S g\,\dfrac{(q-q_0)}{|q-q_0|}dSf(q_0) \\[1.5em] & = &f(q_0)\displaystyle\int_S\dfrac{dS}{|q-q_0|^3}\\[1.5em] & = & f(q_0)\left(2\pi^2\right)\end{array}$

Dividing both sides by $2\pi^2$ yields the exact statement of the result. $\,\square$

The above is part of a (rather long) paper I finished last semester that’s in the infancy of a future as a potential pre-print/publication. I’m not going to link to it until I’ve done a bit more with it. Worth noting, too, is that this is the first time I’ve ever used WordPress’s incarnation of LaTeX, meaning there are many kinks to work out and much tinkering to do. I’m hoping that I can fit this blog into my plans for the rest of the semester, though, so maybe that will force me to do more with it.