# Lecture 1: Basics

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# Categories

Category $𝒞$:
• STRUCTURE/DATA (Graph):

• a class of objects:

$\underbrace{\vert 𝒞 \vert}_{\rlap{\text{also denoted by } {\rm ob}\, 𝒞 \text{, or } 𝒞_0}} : Set$
• a “Hom” map, associating to each every pair of objects a morphism/arrow:

$\underbrace{\Hom_𝒞}_{\rlap{\text{ also denoted by } 𝒞(\bullet) \text{, or } 𝒞_1}}: \vert 𝒞 \vert ⟶ \vert 𝒞 \vert ⟶ Set$
• For all objects:

• identity maps:

$\ast \xto { {\rm id}_x} \Hom_𝒞(x, x)$
• composition maps:

$\circ: \Hom_𝒞(y, z) × \Hom_𝒞(x, y) ⟶ \Hom_𝒞(x, z)$
• LAWS/AXIOMS:

• ${\rm id}_x$ is an identity for $\circ$

• the following diagram commutes:

$\begin{xy} \xymatrix{ 𝒞(z, w) × 𝒞(y, z) × 𝒞(x, y)\ar@{->}[d]_{\circ × 1} \ar@{->}[r]^-{1 × \circ} & 𝒞(y, w) × 𝒞(x, y) \ar@{->}[d]^{\circ} \\ 𝒞(z, w) × 𝒞(x, z) \ar@{->}[r]_{\circ} & 𝒞(x, w) } \end{xy}$
Metacategory:

arbitrary class of objects and morphisms, interpretation satisfying the axioms/laws.

Ex: metacategories of all sets, groups, topological spaces, etc…

Category:

interpretation of the category axioms within set theory

NB: a category is a monoid for the product defined as

$E ×_V E ≝ \lbrace \pair g f \; \mid \; g, f ∈ E \text{ and } t f = s g\rbrace$

where

$\begin{xy} \xymatrix{ E \ar@<-3pt>[r]_{ s } \ar@<+3pt>[r]^{ t } & V } \end{xy}$

is a graph.

### $2$-Categories:

• $\Hom_𝒞$ has its values in $Cat$
• the composition and identity maps are functors
• ${\rm id}x$ and ${\rm id}{ {\rm id}_x}$ are identities for the composition functor.

### Arrows-only version

We can do without objects, as identity arrows are in one-to-one correspondence with objects.

Arrows-only version:

• STRUCTURE:
• arrows $f, g, …$
• certain pairs $\pair g f$
• composition operation: $\pair g f ⟼ gf$
• LAWS:
• $k(gf)$ is defined iff $(kg)f$ is, and when they are, they are equal (denoted by $kgf$)
• $kgf$ is defined iff $kg$ and $gf$ are
• for each $f$, there exist identity arrows (i.e. arrows such $u$ that $fu = u$ whenever $fu$ is defined and $uf = u$ whenever $uf$ is defined) $u$ and $u’$ such that $fu$ and $u’f$ are defined.
• the arrows of a metacategory of objects and arrows satisfy the arrow-only axioms

• conversely: the identity arrows of an arrows-only metacategory, taken as objects, satisfy the objects-and-arrows axioms.

## Natural transformations

• Why commutativity? For horizontal composition
• Naturality (defined by the same formula for all objects): example of $V ≅ V^\ast$ (not natural) and $V ≅ (V^\ast)^\ast$ (natural)
• Equivalence of Categories

# Exercises

## Functors

Faithfulness/Fullness. Example: the forgetful functor $U: Ab ⟶ Set$ is faithful but not full.

Let $Grp \xto F Ab$ be a function on objects such that $F(G) = Z(G)$ for all $G$ (center of $G$). Can we extend $F$ to a functor?

Solution: No.

By contradiction: Consider $𝔖_3 xto {ε} (\lbrace ± 1\rbrace, ×)$ (signature) and $(\lbrace ± 1\rbrace, ×) \xto {σ} 𝔖_3$ where $σ(-1) = (1, 2)$.

Take the image by $F$:

• $\lbrace1\rbrace \xto {F ε} (\lbrace ± 1\rbrace, ×)$ (as $𝔖_3$ has a trivial center and $\lbrace ±1 \rbrace$ is abelian)
• $(\lbrace ± 1\rbrace, ×) \xto {F σ} \lbrace1\rbrace$

And:

$ε \circ σ = id_{\lbrace ± 1\rbrace}$

But

$F ε \circ F σ = id_{\lbrace ± 1\rbrace}$

cannot hold, because $F σ$ is not injective (or: $F ε$ is not surjective).

## Group action

An action of a group $G$ on a set $A$:

denoted by $G ⟶ A$, is a homomorphism $φ: G ⟶ Bij(A)$

$φ$ is faithful:

if $Ker \, φ = \lbrace e_G \rbrace$

$φ$ is free:

$∃x ∈ A; \; φ(g)(x)=x ⟹ g = e_G$

$φ$ is transitive:

$∀x, y ∈ A, \; ∃g; \, φ(g)(x)=y$

Exercise: $F ⊣ G: B ⟶ A$ $G$ is full iff $∀b, ∃ f; f \circ ε_b = id_{FG b}$

Exercise: $F ⊣ G: B ⟶ A$ $G$ is faithful iff $ε_B$ is epi

Exercise (Example of Stone Duality): $FinSet^{op} ≅ FinBoolAlg$

Exercise: $id, (\_)^{\ast \ast}: FdVect ⟶ FdVect$

$id \overset{α}{⟹} (\_)^{\ast \ast}$

Exercise: Determinant as a natural transformation.

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