Categories Functors

When to Use

Use this skill when working on categories-functors problems in category theory.

Decision Tree

1. Verify Category Axioms - Objects and morphisms (arrows) defined? - Identity morphism for each object: id_A: A -> A - Composition associative: (f . g) . h = f . (g . h) - Write Lean 4: theorem assoc : (f ≫ g) ≫ h = f ≫ (g ≫ h) := Category.assoc

2. Check Functor Properties - F: C -> D maps objects to objects, arrows to arrows - Preserves identity: F(id_A) = id_{F(A)} - Preserves composition: F(g . f) = F(g) . F(f) - Write Lean 4: theorem comp : F.map (g ≫ f) = F.map g ≫ F.map f := F.map_comp

3. Functor Types - Covariant: preserves arrow direction - Contravariant: reverses arrow direction - Faithful/Full: injective/surjective on Hom-sets - Equivalence: full, faithful, essentially surjective

4. Common Functors - Forgetful functor: forgets structure (e.g., Grp -> Set) - Free functor: left adjoint to forgetful - Hom functor: Hom(A, -) or Hom(-, B) - Power set functor: Set -> Set via X |-> P(X)

5. Verify with Lean 4 - Compiler-in-the-loop: write proof, lake build checks - Mathlib has full category theory library - See: .claude/skills/lean4-functors/SKILL.md for exact syntax

Tool Commands

Lean4_Category

# Lean 4 with Mathlib: import CategoryTheory.Category.Basic

Lean4_Functor

# Lean 4: theorem map_comp (F : C ⥤ D) : F.map (g ≫ f) = F.map g ≫ F.map f := F.map_comp

Lean4_Build

lake build  # Compiler-in-the-loop verification

Cognitive Tools Reference

See .claude/skills/math-mode/SKILL.md for full tool documentation.