5.2 Exponential objects and evaluation morphisms

Exponential objects in category theory generalize function spaces, denoted as B^A. They have a universal property that ensures a unique morphism for any object X and morphism f: X × A → B, establishing a bijective correspondence between certain Hom-sets.

Cartesian closed categories, like Set and Cat, have all finite products and exponential objects. The evaluation morphism ev: B^A × A → B represents function application. Exponential objects are unique up to isomorphism and connect to adjunctions between product and exponential functors.

Exponential Objects in Category Theory

Definition of exponential objects

• Exponential objects generalize function spaces in category theory denoted as $B^A$ for objects A and B in a category C (Set, Top)
• Universal property ensures existence of unique morphism for any object X and morphism f: X × A → B establishing bijective correspondence between Hom(X × A, B) and Hom(X, B^A)
• Cartesian closed categories possess all finite products and exponential objects (Cat, Set)
• Currying transforms function of multiple arguments into sequence of functions ($f(x,y) \to f(x)(y)$)

Construction of evaluation morphisms

• Evaluation morphism ev: $B^A × A → B$ represents function application in the category
• Construction steps:
  1. Start with identity morphism id: $B^A → B^A$
  2. Apply universal property to obtain ev: $B^A × A → B$
• Commutative diagram illustrates relationship between ev and other morphisms

Uniqueness of exponential objects

• Uniqueness theorem states if $B^A$ and C both satisfy universal property, then $B^A ≅ C$
• Proof outline:
  1. Assume two objects satisfying universal property
  2. Construct isomorphisms between them using universal property
  3. Show composition of isomorphisms is identity
• Yoneda lemma relates to uniqueness of representable functors

Connection to adjunctions

• Adjunction pairs functors F: C → D and G: D → C with natural bijection
• Exponential-product adjunction:
  • Left adjoint: product functor (- × A)
  • Right adjoint: exponential functor $(B^-)$
• Natural isomorphism Hom(X × A, B) ≅ Hom(X, $B^A$) establishes correspondence
• Curry-uncurry operations correspond to unit and counit of adjunction
• Cartesian closed categories characterized by existence of this adjunction for all objects