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

1. 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

1. Functor Types

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

1. 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)

1. 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.