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## Hypersurfaces, 4-manifolds, and characteristic classes

In this post we’ll compute the (topological) cohomology of smooth projective (complex) hypersurfaces in $\mathbb{CP}^n$. When $n = 3$ the resulting complex surfaces give nice examples of 4-manifolds, and we’ll make use of various facts about 4-manifold topology to try to say more in this case; in particular we’ll be able to compute, in a fairly indirect way, the ring structure on cohomology. This answers a question raised by Akhil Mathew in this blog post.

Our route towards this result will turn out to pass through all of the most common types of characteristic classes: we’ll invoke, in order, Euler classes, Chern classes, Pontryagin classes, Wu classes, and Stiefel-Whitney classes.

## The free cocompletion I

Let $C$ be a (locally small) category. Recall that any such category naturally admits a Yoneda embedding

$\displaystyle Y : C \ni c \mapsto \text{Hom}(-, c) \in \widehat{C}$

into its presheaf category $\widehat{C} = [C^{op}, \text{Set}]$ (where we use $[C, D]$ to denote the category of functors $C \to D$). The Yoneda lemma asserts in particular that $Y$ is full and faithful, which justifies calling it an embedding.

When $C$ is in addition assumed to be small, the Yoneda embedding has the following elegant universal property.

Theorem: The Yoneda embedding $Y : C \to \widehat{C}$ exhibits $\widehat{C}$ as the free cocompletion of $C$ in the sense that for any cocomplete category $D$, the restriction functor

$\displaystyle Y^{\ast} : [\widehat{C}, D]_{\text{cocont}} \to [C, D]$

from the category of cocontinuous functors $\widehat{C} \to D$ to the category of functors $C \to D$ is an equivalence. In particular, any functor $C \to D$ extends (uniquely, up to natural isomorphism) to a cocontinuous functor $\widehat{C} \to D$, and all cocontinuous functors $\widehat{C} \to D$ arise this way (up to natural isomorphism).

Colimits should be thought of as a general notion of gluing, so the above should be understood as the claim that $\widehat{C}$ is the category obtained by “freely gluing together” the objects of $C$ in a way dictated by the morphisms. This intuition is important when trying to understand the definition of, among other things, a simplicial set. A simplicial set is by definition a presheaf on a certain category, the simplex category, and the universal property above says that this means simplicial sets are obtained by “freely gluing together” simplices.

In this post we’ll content ourselves with meandering towards a proof of the above result. In a subsequent post we’ll give a sampling of applications.

## A transcript of my qualifying exam

I passed my qualifying exam last Friday. Here is a copy of the syllabus and a transcript.

Although I’m sure there are more, I’m only aware of two other students at Berkeley who’ve posted transcripts of their quals, namely Christopher Wong and Eric Peterson. It would be nice if more people did this.

## How to invent intuitionistic logic

(This is an old post I never got around to finishing. It was originally going to have a second half about pointless topology; the interested reader can consult Vickers’ Topology via Logic on this theme.)

Standard presentations of propositional logic treat the Boolean operators “and,” “or,” and “not” as fundamental (e.g. these are the operators axiomatized by Boolean algebras). But from the point of view of category theory, arguably the most fundamental Boolean operator is “implies,” because it gives a collection of propositions the structure of a category, or more precisely a poset. We can endow the set of propositions with a morphism $p \to q$ whenever $p \Rightarrow q$, and no morphisms otherwise. Then the identity morphisms $\text{id}_p : p \to p$ simply reflect the fact that a proposition always implies itself, while composition of morphisms

$\displaystyle (p \Rightarrow q) \wedge (q \Rightarrow r) \to (p \Rightarrow r)$

is a familiar inference rule (hypothetical syllogism). Since it is possible to define “and,” “or,” and “not” in terms of “implies” in the Boolean setting, we might want to see what happens when we start from the perspective that propositional logic ought to be about certain posets and figure out how to recover the familiar operations from propositional logic by thinking about what their universal properties should be.

It turns out that when we do this, we don’t get ordinary propositional logic back in the sense that the posets we end up identifying are not just the Boolean algebras: instead we’ll get Heyting algebras, and the corresponding notion of logic we’ll get is intuitionistic logic.

## The cohomology of the n-torus

The goal of this post is to compute the cohomology of the $n$-torus $X = (S^1)^n \cong \mathbb{R}^n/\mathbb{Z}^n$ in as many ways as I can think of. Below, if no coefficient ring is specified then the coefficient ring is $\mathbb{Z}$ by default. At the end we will interpret this computation in terms of cohomology operations.

## Decisions, decisions

Newcomb’s paradox is the name usually given to the following problem. You are playing a game against another player, often called Omega, who claims to be omniscient; in particular, Omega claims to be able to predict how you will play in the game. Assume that Omega has convinced you in some way that it is, if not omniscient, at least remarkably accurate: for example, perhaps it has accurately predicted your behavior many times in the past.

Omega places before you two opaque boxes. Box A, it informs you, contains $1,000. Box B, it informs you, contains either$1,000,000 or nothing. You must decide whether to take only Box B or to take both Box A and Box B, with the following caveat: Omega filled Box B with \$1,000,000 if and only if it predicted that you would take only Box B.

What do you do?

(If you haven’t heard this problem before, please take a minute to decide on an option before continuing.)

## Cartesian closed categories and the Lawvere fixed point theorem

Previously we saw that Cantor’s theorem, the halting problem, and Russell’s paradox all employ the same diagonalization argument, which takes the following form. Let $X$ be a set and let

$\displaystyle f : X \times X \to 2$

be a function. Then we can write down a function $g : X \to 2$ such that $g(x) \neq f(x, x)$. If we curry $f$ to obtain a function

$\displaystyle \text{curry}(f) : X \to 2^X$

it now follows that there cannot exist $x \in X$ such that $\text{curry}(f)(x) = g$, since $\text{curry}(f)(x)(x) = f(x, x) \neq g(x)$.

Currying is a fundamental notion. In mathematics, it is constantly implicitly used to talk about function spaces. In computer science, it is how some programming languages like Haskell describe functions which take multiple arguments: such a function is modeled as taking one argument and returning a function which takes further arguments. In type theory, it reproduces function types. In logic, it reproduces material implication.

Today we will discuss the appropriate categorical setting for understanding currying, namely that of cartesian closed categories. As an application of the formalism, we will prove the Lawvere fixed point theorem, which generalizes the argument behind Cantor’s theorem to cartesian closed categories.