# The 2nd annual π day anime and mathematics post

「涼宮ハルヒの消失」/「茨乃」

Happy $\pi$ day. Once again, Nadeko will bring us in:

Snowy Mountain Syndrome is the third story in The Rampage of Haruhi Suzumiya, the fifth volume of the light novel. It’s the first story that has yet to be animated. It’s also a story that contains the dread spectre of mathematics.

So our SOS-dan is stuck in a mysterious cabin in the middle of a snowstorm on a mountain. They find a mysterious contraption that has an equation displayed:

$$x-y=(D-1)-z$$

and they are to provide $x$, $y$, and $z$. Koizumi and Kyon are confused, but Haruhi rightly identifies this equation as Euler’s polyhedron formula, which is also very often referred to as just Euler’s formula. If you’re referring to it in context, it doesn’t matter that much, but it’s useful to distinguish between all the other things that Euler discovered, which is a hell of a lot.

First, we should probably go over some basic definitions. When we talk about graphs, we’re not talking about bar graphs or pie charts or the like. We’re also not talking about graphs of polynomials on a cartesian plane or other such functions. Graphs are a mathematical structure which, when drawn, looks like a bunch of circles and lines.

Formally, a graph is a pair $G = (V,E)$ where $V$ is a set of vertices and $E$ is a set of edges. Vertices can be any old thing, but each edge is defined as a pair $(u,v)$ where $u$ and $v$ are vertices in $V$. When we draw graphs, we just draw a vertex as a circle and draw an edge as a line that connects the two vertices it’s defined as.

And that’s it! That’s the most general definition of a graph, which means we can end up with a graph that’s completely empty or a graph that’s just a bunch of vertices with no edges in between them. We can even have multiple edges going in between two vertices. Of course, often times, we’d like to add some more constraints, depending on what we want to do with our graphs. Very often, we’d like to restrict the number of edges between two vertices to one and that’s what we’ll do.

Back to the formula, usually it’s given as $\chi=v-e+f$, where $v$ is the number of vertices, $e$ is the number of edges, $f$ is the number of faces, and $\chi$ is called the Euler characteristic. That makes $x=v$, $y=e$, $f=z$ and $D-1=\chi$. Now, the only thing here that we haven’t seen defined yet is a face. Intuitively, we can see that’s just the space that’s bounded by the edges.

What I find strange is the explanation in the novel that $D$ stands for the dimension of the polyhedra. As far as I know, this only works in the three-dimensional case for platonic solids. Once we generalize the structures to other kinds of polyhedra and topological surfaces, that analogy breaks down.

Anyhow, the way the formula is applied in the book is the use that I’m most familiar with, which is as a property of a planar graph. For planar graphs, $\chi=2$. In the novel, they deduce that $\chi=1$ since $D=2$ and that only works because they didn’t count the large face outside of the edges as a face, which we usually do.

But what is a planar graph? Well, if you go back to our definition of a graph, you might notice that all we’ve done is said that it’s a bunch of vertices and edges. We’ve said nothing about how to draw a graph. Usually, we represent vertices as circles and edges as lines in between those circles, but other than that, there’s really nothing telling you what order to draw your circles in or whether your lines have to be completely straight or not or how far apart everything has to be. How you choose to represent your graph is up to you, although if you draw your graph weirdly, you might make the people trying to read it angry.

Informally, a planar graph is a graph that you can draw with none of the edges crossing each other. This seems like a kind of silly thing to be worried about, because it seems like you could just keep on drawing a graph until it works out. Well, for edges with a lot of vertices and edges, it’s not obvious and even for really small graphs. For instance:

At a glance, it doesn’t look like the drawing on the right is planar, but all we have to do is drag one of the vertices into the middle to get the drawing on the left and it turns out they’re both the same graph, $K_4$, the complete graph of order 4.

That’s where Euler’s formula comes in really handy. It gives us a way of figuring out whether or not our graph is planar or not without having to fiddle around with placing edges properly and stuff. You already know how many vertices and edges you’ve got, so all you need to do is make sure you’ve got the right number of faces.

So it’s probably pretty clear at this point that you can’t draw every graph without the edges crossing. We can say something interesting about those graphs too, which just turns out to be another characterization of planar graphs, but oh well. But first, we have to introduce the concept of graph minors.

Suppose we have a graph $G=(V,E)$ and an edge $e=(u,v) \in E(G)$. If we contract the edge $e$, we essentially merge the two vertices into a new vertex, let’s call it $w$, and every edge that had an endpoint at $u$ or $v$ now has $w$ as the corresponding endpoint. Then a graph $H$ is a graph minor of $G$ if we can delete and contract a bunch of edges in $G$ to get $H$ (or a graph that’s isomorphic to $H$).

It turns out that every non-planar graph can be reduced to a minor of one of two graphs. The first is $K_5$, the complete graph of order 5:

The second is $K_{3,3}$, the complete bipartite graph 3,3:

These two graphs are the smallest non-planar graphs, otherwise we’d be able to reduce them further to get another non-planar graph. Like I mentioned before, this is a characterization for planar graphs too, since a planar graph can’t contain a $K_5$ or $K_{3,3}$ minor.

I guess I’ll end by saying that graphs are hella useful, especially in computer science. A lot of people complain about never using math like calc ever. If you’re a developer, you’ll run into graphs everywhere. It’s pretty amazing how many structures and concepts can be represented by a bunch of circles and lines.

# 12 Days VII: I want to be a superhero

「Mage Killer」/「三輪」

The first time I read Fate/Zero, I got through about half of the prologue before forgetting about it. That was before I read Fate/stay night.

One of the great things about Fate/Zero is that the particular Holy Grail War it covers is serious business. Instead of having a bunch of high school students kind of flail about, you have some powerful magi all scheming against each other. These dudes know what they’re doing. And actually, that all of these guys basically off each other one way or another kind of explains why we’re left with high school students ten years later.

The main draw of Fate/zero for me has always been that I’d heard it’s particularly brutal and that there was a magus going around ruining everyone with the help of modern technology. After reading Fate/stay night, Kiritsugu’s character became much, much more interesting. As much as people seem to think that Rider is the best (and I’ll admit he is pretty fantastic), Kiritsugu has always been the most compelling character to me.

Those who are familiar with Fate/ will know that it’s all about dealing with ideals, whether it’s defending your ideals, sorting out your ideals, or having your ideals challenged. It’s kind of an easy subject to bring up when fighting for the Holy Grail. Fate/stay night is essentially about Shirou sorting out his ideals. In Fate/zero, we have Kiritsugu, who has the same ideals as Shirou, but ends up choosing a vastly different way of realizing them.

This combined with the high calibre of opponents means that there is a ton of cool stuff that goes down in this story. The fact that these are people prepared for the Grail War and not high school students not only means that their fights are better just because they’re better, but they’re also much better prepared. Calling the Grail War a war in Fate/stay night always seemed a bit silly to me, but in Fate/zero, I think there are enough casualties and heavy weaponry to justify it.

Of course, that’s all after I went through Fate/stay night. So what drove me to try and read the books the first time? Well, I’d just finished playing Saya no Uta at the time. Saya no Uta probably remains one of the most horrifying and disturbing things I’ve read. So obviously, I was looking for more in that vein and found this light novel that the same guy wrote.

On the physical horror front, I think Team Caster’s got it covered. In terms of emotional despair, we’ve got everyone else who’s connected to the events of Fate/stay night. It’s mentioned somewhere in the notes of the first volume that there is basically no happy ending for Fate/zero given what we know plays out in Fate/stay night. Basically, everyone involved bets the house on winning the Grail and ends up ruining their childrens’ lives in the process.

It’s going to be a long three months once the first half of Fate/zero is finished. I guess there’s uncensored Team Caster fun times to look forward to in March, though.

# 12 Days III: Peaceful island, serene man

「静雄無双」/「巖本英利/旧PNス・タンリー」

I’d planned to write about Durarara!! during the whole twelve days of anime thing, but I dropped it at the last minute to make room for Utena and Star Driver. But now, you’re going, wait a second, there was no Durarararararara!! this year, how can it be a special moment in anime for this year?

Well, you see, it turns out a chunk of the light novels was decently far along. I went through books four through six, which, if we allow ourselves to dream a bit, would correspond to the second season. I liked the anime, but I think everyone is in agreement that the second half is weaker, although whether it was still good or terrible is up for debate. The nice thing about the three books immediately after is that the focus is not on Anri or Kida, so you won’t have to worry about them being a downer in your anime about crazy and exciting things in the big city.

So it should please everyone that a second season of the anime focuses much, much more heavily on everyone’s favourite really angry guy, Heiwajima Shizuo. The fourth book goes back to the Durarara/Baccano standard of telling a story with about a thousand different threads somehow coming together at the end. The fifth and sixth books form one story in which Shizuo is the focus in the same way that Mikado, Anri, and Kida were the main characters of their respective arcs. We’ll also see some familiar faces. An interesting thing about the fourth book is the return of some characters that you may or may not remember and the introduction of several new ones.

Shizuo was a really great side character before. He shows up, gets trolled by Izaya, and proceeds to destroy things, usually with other things. The focus that books 5 and 6 puts on him is great because it develops him from the guy that can be described by the previous sentence into a legitimate badass. I feel like he’s always been thoughtful and that his only real problem is getting pissed off really easily, so I enjoyed watching him put his talents to use in a more focused way.

The thing that I enjoy about Ryohgo Narita’s stuff is his ability to launch a million seemingly unrelated characters and plot threads and be able to tie them together. The one-volume version of that is very neat to read through, but I have a feeling that there’s some longer term stuff that he’s set up in the early books that will come back in a big way and there are already hints of that at the end of the sixth book. This isn’t unlike the Haruhi series, where seemingly inconsequential details come back later in an earthshattering way, although, here the pieces are bigger, I guess.

Really looking forward to that second season.