Category Master

Expert guidance for rigorous categorical reasoning, proofs, and constructions in pure mathematics.

Core Principles

Set-Theoretic Foundations

Size distinctions (essential for avoiding paradoxes):

• Small set: Element of a fixed Grothendieck universe 𝒰
• Small category: Ob(𝒞) and all Hom-sets are small (elements of 𝒰)
• Locally small category: Each Hom(A,B) is small, but Ob(𝒞) may be a proper class
• Large category: Even some Hom-sets may be proper classes

Grothendieck universes: Sets closed under standard operations (pairing, power set, unions), satisfying axioms that enable treating "all small sets" as a category without Russell-type paradoxes.

Practical implications:

• The category Set of all sets is not small; working in Set requires 𝒰
• Yoneda embedding 𝒞 → [𝒞^op, Set] requires 𝒞 locally small
• Functor categories [𝒞, 𝒟]: if 𝒞 small and 𝒟 locally small, then [𝒞, 𝒟] is locally small
• Adjunctions F ⊣ G: natural bijection Hom(F(A), B) ≅ Hom(A, G(B)) requires local smallness

Universe hierarchy (for categories of categories):

• When working with Cat, need 𝒰 ∈ 𝒰' ∈ 𝒰'' ...
• Cat(𝒰) = category of 𝒰-small categories (lives in 𝒰')
• Enables discussing functors between Cat and other 2-categories

Foundation conventions: Unless stated otherwise, assume locally small categories and work within a fixed universe 𝒰 for small sets.

Precision and Rigor

• Always state precise mathematical definitions using proper notation
• Verify universal properties and coherence conditions explicitly
• Check commutative diagrams for all naturality and functoriality requirements
• Cite theorem names and formalize all proof steps
• Be explicit about componentwise vs functor-level operations

Categorical Thinking

• Identify universal constructions (limits, colimits, adjunctions) as primary tools
• Reason arrow-theoretically: prioritize morphisms over internal structure
• Seek canonical isomorphisms and natural transformations
• Apply duality systematically (op-categories, contravariant functors)

Abstraction Levels

Level 0 (Sets): Objects as sets, morphisms as functions
Level 1 (Categories): Objects in categories, morphisms between them, functors between categories
Level 2 (2-Categories): Natural transformations as 2-cells, 2-functors
Level n (Higher): n-cells and coherence at all dimensions

Foundational Structures

1. Categories and Functors

Category Definition: A category 𝒞 consists of:

• Class of objects Ob(𝒞)
• For each A,B ∈ Ob(𝒞), a set of morphisms Hom(A,B)
• Composition: ∘ : Hom(B,C) × Hom(A,B) → Hom(A,C) satisfying associativity
• Identity morphisms id_A ∈ Hom(A,A) satisfying left/right identity laws

Size distinctions:

• Small category: Both Ob(𝒞) and all Hom-sets are small (sets, not proper classes)
• Locally small category: Hom(A,B) is a set for all A,B (even if Ob(𝒞) is a proper class)
• Most categories in practice are locally small (e.g., Set, Grp, Top)
• Size matters for avoiding set-theoretic paradoxes and for functor categories

Functor Definition: F: 𝒞 → 𝒟 maps objects to objects, morphisms to morphisms, preserving:

• Identities: F(id_A) = id_{F(A)}
• Composition: F(g ∘ f) = F(g) ∘ F(f)

Size note: For small 𝒞 and locally small 𝒟, the functor category [𝒞, 𝒟] is well-defined and locally small.

Verification Template:

To prove F is a functor:
1. Define F on objects: F(A) = ...
2. Define F on morphisms: F(f: A → B) = ...
3. Check identity: F(id_A) = ... = id_{F(A)} ✓
4. Check composition: F(g ∘ f) = ... = F(g) ∘ F(f) ✓

2. Natural Transformations

Definition: α: F ⇒ G between functors F,G: 𝒞 → 𝒟 assigns to each A ∈ 𝒞 a morphism α_A: F(A) → G(A) such that for all f: A → B:

F(A) --α_A--> G(A)
 |             |
F(f)         G(f)
 |             |
 v             v
F(B) --α_B--> G(B)

This diagram commutes: G(f) ∘ α_A = α_B ∘ F(f) (naturality square)

Verification Template:

To prove α is natural:
1. Define components: α_A: F(A) → G(A) for each A
2. For each morphism f: A → B, verify:
   G(f) ∘ α_A = α_B ∘ F(f)
3. Compute both paths and confirm equality

3. Adjunctions

Definition: F: 𝒞 ⇄ 𝒟 : G form an adjunction F ⊣ G if there exist natural transformations:

• η: id_𝒞 ⇒ GF (unit)
• ε: FG ⇒ id_𝒟 (counit)

Triangle Identities (⚠️ COMPONENTWISE - Critical Notation):

For each object A ∈ 𝒞:

ε_{F(A)} ∘ F(η_A) = id_{F(A)}  (left triangle)

For each object B ∈ 𝒟:

G(ε_B) ∘ η_{G(B)} = id_{G(B)}  (right triangle)

⚠️ CRITICAL NOTATION WARNING: ❌ INCORRECT: Writing "(ε_F ∘ F(η)) = id_F" treats natural transformations as if they were functors
✓ CORRECT: For each A ∈ 𝒞: ε_{F(A)} ∘ F(η_A) = id_{F(A)}
✓ CORRECT: For each B ∈ 𝒟: G(ε_B) ∘ η_{G(B)} = id_{G(B)}

These are equations between morphisms in 𝒟 and 𝒞 respectively, NOT equations between functors. The subscripts index components of natural transformations. Each component must be verified individually. Sloppy notation conflates natural transformations with functors, obscures the componentwise structure, and causes errors in Beck's monadicity theorem and size considerations in large categories.

Equivalent characterization: Natural bijection

φ_{A,B}: Hom_𝒟(F(A), B) ≅ Hom_𝒞(A, G(B))

natural in both A and B, with φ and φ⁻¹ inverse bijections.

Relationship between characterizations:

• Given unit/counit: φ(f: F(A) → B) = G(f) ∘ η_A
• Given hom-bijection: η_A = φ⁻¹(id_{G(F(A))}), ε_B = φ(id_{F(G(B))})

Size considerations: The hom-bijection requires 𝒞, 𝒟 locally small (otherwise Hom-sets are proper classes). This is essential for:

• Yoneda lemma applications (representability)
• Beck's monadicity theorem (requires local smallness)
• Kan extensions as adjoints

Monadicity context: If U: 𝒟 → 𝒞 creates coequalizers of U-split pairs and has a left adjoint F, then 𝒟 ≃ 𝒞^T where T = UF is the induced monad (Beck's theorem). Verifying componentwise triangle identities is essential for proving monadicity in practice.

Verification Template:

To prove F ⊣ G:
Method 1 (unit-counit):
1. Define η_A: A → GF(A) for all A ∈ 𝒞
2. Define ε_B: FG(B) → B for all B ∈ 𝒟
3. Verify naturality of η: for f: A → A', G(F(f)) ∘ η_A = η_A' ∘ f
4. Verify naturality of ε: for g: B → B', g ∘ ε_B = ε_B' ∘ F(G(g))
5. Check left triangle: for each A, ε_{F(A)} ∘ F(η_A) = id_{F(A)}
6. Check right triangle: for each B, G(ε_B) ∘ η_{G(B)} = id_{G(B)}

Method 2 (hom-isomorphism):
1. Define φ: Hom_𝒟(F(A), B) → Hom_𝒞(A, G(B))
2. Define φ⁻¹: Hom_𝒞(A, G(B)) → Hom_𝒟(F(A), B)
3. Verify φ ∘ φ⁻¹ = id and φ⁻¹ ∘ φ = id
4. Verify naturality in A: for f: A' → A, φ_{A,B}(h ∘ F(f)) = φ_{A',B}(h) ∘ f
5. Verify naturality in B: for g: B → B', φ_{A,B'}(g ∘ h) = G(g) ∘ φ_{A,B}(h)

Common adjunctions:

• Free-forgetful: F: Set ⇄ Grp : U (free group ⊣ underlying set)
• Tensor-hom: -⊗A ⊣ Hom(A,-) in monoidal closed categories
• Direct/inverse image: f* ⊣ f_* for continuous f: X → Y (sheaf theory)
• Quantifiers: ∃_f ⊣ f* ⊣ ∀_f in categorical logic

4. Limits and Colimits

Limit: Given diagram D: 𝒥 → 𝒞, a limit is a terminal cone to D

• Cone: object L with morphisms π_j: L → D(j) making all diagrams commute
• Universal property: for any other cone (K, ψ_j), ∃! u: K → L factoring through π_j

Common Examples:

• Terminal object (limit of empty diagram)
• Products A × B (limit of discrete diagram {A, B})
• Equalizers (limit of parallel pair f,g: A ⇉ B)
• Pullbacks (limit of cospan A → C ← B)

Colimit: Dual notion (initial cocone from D)

Verification Template:

To verify L is a limit of D:
1. Specify cone morphisms π_j: L → D(j)
2. Check commutativity: for all α: j → k in 𝒥,
   D(α) ∘ π_j = π_k
3. Universal property: given any cone (K, ψ_j),
   construct unique u: K → L such that π_j ∘ u = ψ_j
4. Verify uniqueness of u

5. Monoidal Categories

Definition: (𝒞, ⊗, I) consists of:

• Bifunctor ⊗: 𝒞 × 𝒞 → 𝒞 (tensor product)
• Unit object I
• Natural isomorphisms:
  • α: (A ⊗ B) ⊗ C → A ⊗ (B ⊗ C) (associator)
  • λ: I ⊗ A → A (left unitor)
  • ρ: A ⊗ I → A (right unitor)

Satisfying pentagon (Mac Lane coherence) and triangle axioms.

Pentagon Axiom: For A,B,C,D composable, this diagram commutes:

((A⊗B)⊗C)⊗D --α--> (A⊗B)⊗(C⊗D)
     |                     |
     α                     α
     |                     |
     v                     v
(A⊗(B⊗C))⊗D          A⊗(B⊗(C⊗D))
     |                     |
    α⊗id                 id⊗α
     |                     |
     v                     v
A⊗((B⊗C)⊗D) ----α----> A⊗(B⊗(C⊗D))

Triangle Axiom: For A,B, relates α, λ, ρ

Symmetric monoidal: Add braiding β: A ⊗ B → B ⊗ A satisfying hexagon axioms

Closed monoidal: Add internal hom [A, B] with natural isomorphism:

Hom(A ⊗ B, C) ≅ Hom(A, [B, C])

String diagrams: Graphical calculus for monoidal categories

• Objects → Wires (vertical lines)
• Morphisms → Boxes/nodes
• Composition → Vertical stacking
• Monoidal product → Horizontal juxtaposition
• Topology determines equality (isotopy invariance)

6. Enriched Categories

Definition: 𝒞 enriched over monoidal category (𝒱, ⊗, I) has:

• Hom-objects Hom(A,B) ∈ Ob(𝒱) (not sets!)
• Composition morphism: Hom(B,C) ⊗ Hom(A,B) → Hom(A,C) in 𝒱
• Identity morphism: I → Hom(A,A) in 𝒱

Satisfying associativity and identity coherence in 𝒱.

Common enrichments:

• Poset-enriched: Hom(A,B) ∈ {⊥, ⊤} (preorders)
• Ab-enriched: Hom(A,B) are abelian groups (preadditive categories)
• Cat-enriched: Hom(A,B) are small categories (2-categories)

7. Elementary Toposes

❌ INCORRECT DEFINITION (common error): "Cartesian closed category with finite colimits"

✓ CORRECT DEFINITION: An elementary topos 𝓔 is a category with:

1. All finite limits

   • Terminal object 1
   • Binary products A × B
   • Equalizers of parallel pairs f,g: A ⇉ B

2. Exponentials (cartesian closedness)

   • For all A,B ∈ 𝓔, exists B^A with evaluation ev: B^A × A → B
   • Universal property: For any f: C × A → B, unique λ(f): C → B^A with f = ev ∘ (λ(f) × id_A)

3. Subobject Classifier Ω

   • Distinguished object Ω with morphism true: 1 → Ω
   • Universal property: For every monic m: S ↪ A, exists unique χ_m: A → Ω (characteristic morphism of m) making:

   S --------!-------> 1
   |                   |
   m                  true
   |                   |
   v                   v
   A ------χ_m-------> Ω
   

   a pullback square.

   • Conversely: Every such pullback defines a monic
   • Establishes bijection: {monics into A} ↔ {morphisms A → Ω}

Key Consequences:

• Power objects exist: Ω^A ≅ Sub(A) (subobjects of A)
• Finite colimits are derivable (via internal logic, NOT axioms!)
  • Initial object, coproducts, coequalizers constructed from limits, exponentials, and Ω
• Internal logic is intuitionistic (Mitchell-Bénabou language)
  • Ω acts as "truth values" object
  • Logical operations: ∧, ∨, →, ⊥, ⊤ are morphisms in 𝓔
  • NOT Boolean in general (Law of excluded middle fails)

Examples:

1. Set (prototypical topos)

   • Ω = {0, 1} or {false, true}
   • true: {*} → {0,1} sends * ↦ 1
   • χ_S: A → {0,1} is indicator function: χ_S(a) = 1 iff a ∈ S

2. Sh(X) (sheaves on topological space X)

   • Ω = "sheaf of truth values" (open sets with restrictions)
   • Ω(U) = {opens V ⊆ U}
   • Realizes topological intuition: "truth varies by location"

3. Set^{𝒞^op} (presheaves on category 𝒞)

   • Ω(C) = {sieves on C} (right-closed subfunctors of Hom(−, C))
   • Basis for Grothendieck topology

❌ NON-EXAMPLE (Counterexample):

FinSet (category of finite sets):

• ✓ Has finite limits (products, equalizers)
• ✓ Is cartesian closed (exponentials exist: B^A is set of functions)
• ✓ Has finite colimits (unions, coproducts)
• ✗ Does NOT have subobject classifier

Why FinSet fails:

• Would need Ω such that {subsets of A} ↔ {functions A → Ω}
• For infinite A, this requires Ω to have size |P(A)|
• But FinSet requires Ω to be finite!
• Contradiction: Cannot represent arbitrary subsets of infinite sets with finite Ω

This counterexample proves that "cartesian closed + finite colimits" is insufficient for a topos. The subobject classifier Ω is essential and cannot be derived from other axioms.

Grothendieck Toposes (Related but distinct):

A Grothendieck topos is a category equivalent to Sh(𝒞, J) (sheaves on site):

• Site = (𝒞, J) where J is Grothendieck topology (coverage)
• J assigns to each C a collection of covering sieves
• Sheaf condition: Gluing axiom for covers

Relationship:

• Every Grothendieck topos is an elementary topos
• NOT conversely: Elementary toposes need not come from sites
• Grothendieck version emphasizes sheaf theory, sites, descent
• Elementary version emphasizes logic, power objects, internal language

Internal logic applications:

• Topos structure enables intuitionistic logic via Ω (propositions as subobjects of 1)
• Mitchell-Bénabou language for higher-order reasoning
• Kripke-Joyal semantics for forcing (truth relative to objects as "stages")

Applications:

• Algebraic geometry (étale topos)
• Categorical logic (intuitionistic type theory)
• Forcing and independence results
• Synthetic differential geometry

8. Higher Categories

2-Category (strict): Categories with 2-cells (natural transformations)

• Objects, 1-cells (functors), 2-cells (natural transformations)
• Horizontal composition (α ∗ β) and vertical composition (α · β)
• Interchange law: (α' · α) ∗ (β' · β) = (α' ∗ β') · (α ∗ β)
• Pasting diagrams: All compositions of 2-cells commute when boundaries match
• String diagrams: Graphical calculus where topology determines equality

Bicategory (weak 2-category): Composition and identities hold up to coherent isomorphisms

• Associator: α_{f,g,h}: (h∘g)∘f ⇒ h∘(g∘f) (invertible 2-cell, not identity)
• Left/right unitors: λ_f: id_B ∘ f ⇒ f and ρ_f: f ∘ id_A ⇒ f
• Pentagon coherence: For composable f,g,h,k, the pentagon of associators commutes:

  ((k∘h)∘g)∘f ---α---> (k∘h)∘(g∘f)
       |                     |
       α                     α
       |                     |
       v                     v
  (k∘(h∘g))∘f           k∘(h∘(g∘f))
       |                     |
      α∘id                 id∘α
       |                     |
       v                     v
  k∘((h∘g)∘f) ----α----> k∘(h∘(g∘f))
  

• Triangle coherence: For f,g, the triangle identity diagram commutes:

  (f∘id)∘g ----α----> f∘(id∘g)
      \                 |
       \                |
     ρ∘id           id∘λ
         \              |
          \             v
           ----===----> f∘g
  

• Strictification: Every bicategory is biequivalent to a strict 2-category (Mac Lane-Paré coherence)

Pasting Diagrams (computational tool for 2-categories):

    A --f--> B --g--> C
    |        |        |
    h        k        l
    |   α    |   β    |
    v        v        v
    D --m--> E --n--> F

Composition rules:

• Vertical (∘): Compose along objects (α;β when target of α = source of β)
• Horizontal (⊗): Compose along 1-morphisms (α⊗β in parallel)
• Interchange: (α⊗β);(γ⊗δ) = (α;γ)⊗(β;δ) when composable

Gray-categories (semi-strict 3-categories): Composition strictly associative, but interchange laws hold up to isomorphism

• Intermediate between strict and weak 3-categories
• Gray tensor product: Monoidal structure on 2-Cat encoding "lax composition"

Tricategories (weak 3-categories):

• Objects, 1-cells, 2-cells, 3-cells (modifications)
• Pentagonator: 3-cell witnessing coherence of four associators (Mac Lane's pentagon one dimension up)
• Every tricategory is triequivalent to a Gray-category (Power's coherence theorem)

n-Categories: Generalizes to n levels of cells

• Strict n-category: All compositions strictly associative, identities strict, interchange laws hold as equalities
• Weak n-category: Associativity/identity up to (k-1)-cells for k ≤ n, with coherence axioms at each level
• Semistrict: Some structure strict, some weak (e.g., composition strict but identity weak)

∞-Categories (models for homotopy-coherent mathematics):

• Quasi-categories (Joyal-Lurie): Simplicial sets with inner horn fillers (Λᵏₙ → X extends to Δⁿ → X for 0 < k < n)
  • 0-simplices: objects
  • 1-simplices: morphisms
  • 2-simplices: homotopies/commutative triangles
  • n-simplices: higher coherence data
  • Models (∞,1)-categories (all n≥2 morphisms are equivalences)
• Complete Segal spaces (Rezk): Simplicial spaces satisfying Segal and completeness conditions
• Simplicially enriched categories: Categories enriched over sSet (Dwyer-Kan model)
• All models equivalent via Quillen equivalences of model structures

Orientals: Simplicial sets O[n] encoding pasting schemes

• O[0] = point
• O[1] = arrow
• O[2] = commutative triangle
• O[n] = coherent n-fold composition

Strictification Theorems and Limitations:

• n ≤ 1: All weak categories equivalent to strict (posets, groupoids)
• n = 2: Bicategories ≃ strict 2-categories (Mac Lane-Paré)
• n = 3: Tricategories ≃ Gray-categories (Power), partial strictification
• n ≥ 4: ❌ Simpson's conjecture disproven (Lack et al.) - weak n-categories strictly richer than strict ones; full strictification IMPOSSIBLE
• ∞-categories: Inherently weak, no global strictification possible

Coherence Theorems: In weak n-categories, all pasting diagrams of canonical cells commute up to canonical higher cells. This allows proof simplification by "assuming strictness locally."

Mac Lane Coherence Theorem (Monoidal Categories):

• In monoidal category, "all diagrams of canonical isomorphisms commute"
• Precise statement: Every diagram built from α, λ, ρ (associator, unitors) commutes
• Equivalently: Free monoidal category on one object is poset

9. Operads

Non-Symmetric Operads (Basic Definition):

A non-symmetric operad 𝓞 in symmetric monoidal category (𝒱,⊗,I) consists of:

1. Objects: 𝓞(n) ∈ 𝒱 for each arity n ≥ 0

   • 𝓞(n) represents "n-ary operations"

2. Composition: Multilinear maps γ: 𝓞(k) ⊗ 𝓞(n₁) ⊗ ... ⊗ 𝓞(nₖ) → 𝓞(n₁ + ... + nₖ)

   Intuition: Given k-ary operation and operations for each input, compose to get (n₁+...+nₖ)-ary operation

3. Unit: Element id ∈ 𝓞(1) (identity operation)

Axioms:

1. Associativity: Iterated compositions associative
2. Unit: Composing with id doesn't change operation

Examples (non-symmetric):

• Associative operad 𝓐ss: 𝓐ss(n) = 𝟙 (single n-ary operation)
  • Encodes associative (non-commutative) algebras
• Endomorphism operad: End_X(n) = Hom(X^⊗n, X)
  • Operations are actual morphisms in 𝒱

Symmetric Operads (With Permutations):

A symmetric operad has additional structure:

4. Symmetric group actions: Right action of Σ_n on 𝓞(n)

   • σ ∈ Σ_n acts on 𝓞(n)
   • Intuition: Permute inputs of n-ary operations

5. Equivariance: Composition respects permutations

Examples (symmetric):

• Commutative operad 𝓒om: 𝓒om(n) = 𝟙 with Σ_n acting trivially
  • Encodes commutative algebras
• E_∞ operad: Contractible Σ_n-spaces
  • "Maximally commutative" up to homotopy

Key Distinction:

Non-symmetric: Order of inputs matters (e.g., matrix multiplication)
Symmetric: Can permute inputs (e.g., addition is commutative)

When to Use Which:

• Non-symmetric: Sufficient for associative structures, loop spaces, A_∞-algebras in homotopy theory
• Symmetric: Required for commutative structures, recognition principle (May 1972), E_∞-ring spectra

Algebras over an Operad:

An algebra over operad 𝓞 is:

• Object A ∈ 𝒱
• Structure maps: 𝓞(n) ⊗ A^⊗n → A for each n
• Satisfying associativity, unit, equivariance (if symmetric)

Examples:

• Algebra over 𝓐ss = associative algebra (non-commutative)
• Algebra over 𝓒om = commutative algebra
• Algebra over E_∞ = E_∞-algebra (homotopy commutative)

Operad Variants:

1. Colored (Multi-Sorted) Operads:

   • Operations typed: 𝓞(c₁,...,cₙ;d) for colors c₁,...,cₙ,d
   • Composition respects typing
   • Example: Categories as colored operads (objects = colors)

2. Cyclic Operads:

   • Additional cyclic Cₙ-action on 𝓞(n)
   • Captures "trace" or "inner product" operations
   • Example: Modular operads in string topology

3. ∞-Operads (Homotopy-Coherent):

   • Weak/higher versions (Lurie, Cisinski-Moerdijk)
   • Operad structure up to coherent homotopy
   • Model ∞-categories as ∞-operads

4. PROPs (Products and Permutations):

   • Generalize operads: 𝓟(m,n) (m outputs, n inputs)
   • Capture operations like tensor product (2 outputs)
   • Wiring diagrams for composition

Connection to monads: Operads in Set correspond to certain finitary monads on Set (via free algebra construction)

Applications:

• Algebra: Universal algebra, Lie algebras, Poisson algebras
• Topology: Recognition principle (May), loop spaces, iterated loop spaces
• Homotopy Theory: A_∞, E_∞ structures in stable homotopy theory
• Geometry: Moduli spaces (Deligne-Mumford, Kontsevich)
• Physics: Feynman diagrams, BV quantization

Historical Note:

• Non-symmetric operads: Foundational, sufficient for many classical examples
• Symmetric operads: Enable commutativity, Eckmann-Hilton argument
• Recognition principle requires symmetric structure (May, 1972)

Proof Techniques

Diagram Chasing

1. Identify the diagram and what needs to be proven
2. Label all objects and morphisms explicitly
3. Use commutativity systematically
4. Apply universal properties to construct/identify morphisms
5. Verify uniqueness conditions

Universal Property Arguments

Pattern:

Given: Universal object U with property P
To show: U satisfies Q
Proof:
1. Assume X also satisfies P
2. By universality, ∃! u: U → X
3. Show u demonstrates Q
4. Uniqueness ensures Q is canonical

Yoneda Lemma Applications

Yoneda Lemma: Nat(Hom(A, -), F) ≅ F(A) naturally

Use to:

• Prove isomorphisms by checking on representables
• Show functors are isomorphic by showing hom-sets are
• Construct morphisms via natural transformations

Coherence Theorems

For monoidal/enriched categories, all diagrams of canonical isomorphisms commute.

Verification approach:

1. Reduce to checking Mac Lane's pentagon and triangle
2. All other coherences follow automatically
3. Apply coherence theorem to simplify proofs

Common Constructions

Comma Categories

Given functors F: 𝒞 → ℰ, G: 𝒟 → ℰ, the comma category (F ↓ G) has:

• Objects: triples (C, D, f: F(C) → G(D))
• Morphisms: pairs (u: C → C', v: D → D') making the obvious square commute

Special cases:

• Slice category 𝒞/A when G = const_A
• Coslice category A/𝒞 when F = const_A

Kan Extensions

Given F: 𝒞 → 𝒟 and K: 𝒞 → ℰ:

• Left Kan extension Lan_K F is left adjoint to precomposition with K
• Right Kan extension Ran_K F is right adjoint to precomposition with K

Formula: (Lan_K F)(E) = colim_{K(C) → E} F(C)

Monadicity

A functor U: 𝒟 → 𝒞 is monadic if 𝒟 ≃ 𝒞^T for some monad T on 𝒞.

Beck's monadicity theorem provides conditions (U creates coequalizers of U-split pairs, etc.)

Working with This Skill

For Proving Theorems

1. State theorem precisely with all hypotheses (including size conditions)
2. Identify relevant universal properties and their variance
3. Draw all necessary commutative diagrams with explicit objects/morphisms
4. Apply proof techniques systematically (diagram chasing, Yoneda, coherence)
5. Verify all coherence conditions and naturality squares componentwise
6. Check triangle identities for adjunctions object-by-object

For Constructing Categorical Frameworks

1. Define objects and morphisms explicitly with size specifications
2. Verify category axioms (associativity, identity, size closure)
3. Identify universal constructions (limits, adjunctions) and prove universality
4. Check functoriality: preserve identities and composition
5. Verify naturality: all relevant squares commute
6. Establish coherence for higher structures (pentagon, triangle axioms)
7. Address set-theoretic foundations (universes, local smallness)

For Technical Verification

Always verify componentwise:

• Natural transformations: check α_A for each object A, then naturality square for each morphism
• Adjunction triangles: verify ε_{F(A)} ∘ F(η_A) = id_{F(A)} for each A individually
• Unit/counit naturality: check commutativity for each morphism explicitly

Size considerations checklist:

• Are all categories locally small (Hom-sets are sets)?
• For functor categories [𝒞,𝒟], is 𝒞 small?
• Do colimits/limits stay within the universe?
• Are representables well-defined (local smallness ensures this)?

Coherence verification:

• Monoidal categories: verify pentagon and triangle, invoke Mac Lane coherence for rest
• Symmetric monoidal: add hexagon axioms for braiding
• Bicategories: verify pentagon for associator, triangles for unitors
• Higher categories: check coherence at each dimension

Common pitfalls to avoid:

• Confusing functor-level notation with componentwise structure in adjunctions
• Assuming all weak n-categories strictify (false for n ≥ 4)
• Omitting size checks (causes subtle paradoxes)
• Claiming "Cartesian closed + colimits = topos" (need subobject classifier!)
• Assuming all operads are symmetric (non-symmetric variants exist and are fundamental)
• Forgetting to verify naturality in addition to defining components

Reasoning Strategies

Universal property pattern:

Goal: Show object U satisfies property P
1. Assume X also satisfies P
2. By universality of U, ∃! u: U → X (or X → U depending on variance)
3. Show this unique morphism demonstrates P
4. Uniqueness ensures canonicity

Yoneda lemma applications:

• To prove F ≅ G, show Hom(A, F(-)) ≅ Hom(A, G(-)) for all A
• To construct morphism A → B, construct natural transformation Hom(-,A) ⇒ Hom(-,B)
• Representable functors are limits (products, equalizers, pullbacks)

Duality exploitation:

• Every statement about limits has dual about colimits
• Left adjoints are colimit-preserving; right adjoints are limit-preserving
• Work in 𝒞^op to dualize proofs systematically

References and Further Study

Foundational Texts

• Mac Lane, S. Categories for the Working Mathematician (1971, 2nd ed. 1998) - Standard reference for basic category theory, Mac Lane coherence, limits/colimits, adjunctions, Yoneda
• Borceux, F. Handbook of Categorical Algebra (3 volumes, 1994) - Comprehensive treatment: Vol 1 (basic theory), Vol 2 (abelian categories), Vol 3 (categories of sheaves)
• Awodey, S. Category Theory (2010, 2nd ed.) - Modern accessible introduction with emphasis on logic and foundations
• Leinster, T. Basic Category Theory (2014) - Concise modern treatment, excellent for quick reference
• Riehl, E. Category Theory in Context (2016) - Modern pedagogical approach with emphasis on universal properties

Elementary Toposes and Logic

• Johnstone, P.T. Topos Theory (1977) - Classic introduction to elementary toposes
• Johnstone, P.T. Sketches of an Elephant: A Topos Theory Compendium (2002) - Encyclopedic treatment of Grothendieck toposes, sites, geometric logic
• Mac Lane, S. & Moerdijk, I. Sheaves in Geometry and Logic (1992) - Toposes for geometers and logicians, Mitchell-Bénabou language, Kripke-Joyal semantics
• Goldblatt, R. Topoi: The Categorial Analysis of Logic (1984) - Elementary introduction with focus on internal logic

Enriched Categories

• Kelly, G.M. Basic Concepts of Enriched Category Theory (1982, Cambridge LNM 64) - Definitive treatment of V-categories, tensored/cotensored categories, enriched Yoneda
• Borceux, F. & Dejean, D. "Cauchy completion in category theory" (1986) - Enrichment and completion

Higher Categories

• Lurie, J. Higher Topos Theory (2009) - ∞-categories via quasi-categories, fundamental work in homotopy theory
• Lurie, J. Higher Algebra (2017) - ∞-operads, monoidal ∞-categories, algebras
• Leinster, T. Higher Operads, Higher Categories (2004, LMS 298) - Operads, n-categories, comparison of models
• Riehl, E. & Verity, D. Elements of ∞-Category Theory (2022) - Modern foundations using ∞-cosmoi, model-independent approach

Homological Algebra and Derived Categories

• Weibel, C. An Introduction to Homological Algebra (1994) - Standard text for chain complexes, derived functors, spectral sequences
• Gelfand, S. & Manin, Y. Methods of Homological Algebra (2003) - Derived categories, triangulated categories, applications to algebraic geometry
• Kashiwara, M. & Schapira, P. Categories and Sheaves (2006) - Advanced treatment of derived categories and sheaves

Model Categories and Homotopy Theory

• Hovey, M. Model Categories (1999, AMS 63) - Definitive introduction to Quillen model structures
• Hirschhorn, P. Model Categories and Their Localizations (2003) - Advanced topics in model categories
• Dwyer, W. & Spalinski, J. "Homotopy theories and model categories" (1995, Handbook of Algebraic Topology) - Accessible introduction

Monoidal and Symmetric Monoidal Categories

• Joyal, A. & Street, R. "Braided tensor categories" Advances in Mathematics 102 (1993) - Coherence for symmetric monoidal categories, braiding
• Selinger, P. "A survey of graphical languages for monoidal categories" (2011) - String diagrams, graphical calculus
• Etingof, P. et al. Tensor Categories (2015) - Advanced treatment with applications to representation theory

Bicategories and Coherence

• Bénabou, J. "Introduction to bicategories" (1967, LNM 47) - Original definition of bicategories
• Mac Lane, S. & Paré, R. "Coherence for bicategories and indexed categories" (1985) - Strictification theorem
• Lack, S. "A coherent approach to pseudomonads" Advances in Mathematics 152 (2000) - Coherence for bicategories
• Power, J. "A general coherence result" Journal of Pure and Applied Algebra 57 (1989) - Tricategory coherence

Operads

• May, J.P. The Geometry of Iterated Loop Spaces (1972, LNM 271) - Original operadic approach to loop spaces, recognition principle
• Loday, J.-L. & Vallette, B. Algebraic Operads (2012) - Modern comprehensive treatment of operads
• Boardman, J.M. & Vogt, R.M. Homotopy Invariant Algebraic Structures on Topological Spaces (1973, LNM 347) - Operads and homotopy theory

Accessible and Locally Presentable Categories

• Adámek, J. & Rosický, J. Locally Presentable and Accessible Categories (1994, Cambridge LMS 189) - Size issues, colimits, applications
• Makkai, M. & Paré, R. Accessible Categories: The Foundations of Categorical Model Theory (1989, AMS 104) - Model-theoretic approach

Kan Extensions and Representability

• Mac Lane, S. (1971) Chapters VII-X - Kan extensions, pointwise formulas, representable functors
• Riehl, E. Categorical Homotopy Theory (2014, Cambridge) - Kan extensions in homotopy contexts

Foundations and Universes

• Grothendieck, A. "Univers" in SGA 4 (1972) - Original development of Grothendieck universes
• Shulman, M. "Set theory for category theory" (2008, arXiv:0810.1279) - Modern treatment of size issues, including alternatives to universes
• Voevodsky, V. "Univalent Foundations" (ongoing) - Homotopy type theory approach to foundations