Adjoints and Monads

Description: Test your understanding of adjoints and monads in category theory.
Number of Questions: 15
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Tags: category theory adjoints monads
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Given categories (C) and (D), what is the definition of an adjoint pair of functors (F: C \to D) and (G: D \to C)?

  1. For every object (X \in C) and every object (Y \in D), there exists a bijection between the set of morphisms from (F(X)) to (Y) in (D) and the set of morphisms from (X) to (G(Y)) in (C).

  2. For every object (X \in C) and every object (Y \in D), there exists a natural isomorphism between the functor (F) and the functor (G).

  3. For every object (X \in C) and every object (Y \in D), there exists a natural transformation from (F) to (G) and a natural transformation from (G) to (F) such that their composition is the identity natural transformation.

  4. For every object (X \in C) and every object (Y \in D), there exists a natural transformation from (F) to (G) such that its composition with itself is the identity natural transformation.


Correct Option:
Explanation:

An adjoint pair of functors is characterized by the existence of natural transformations between them that satisfy certain properties, including the composition of these natural transformations being the identity natural transformation.

In the context of an adjoint pair of functors (F: C \to D) and (G: D \to C), what is the unit of the adjunction?

  1. A natural transformation (\eta: \text{Id}_C \to G \circ F) such that for every object (X \in C), the component (\eta_X: X \to G(F(X))) is an isomorphism.

  2. A natural transformation (\eta: F \circ G \to \text{Id}_D) such that for every object (Y \in D), the component (\eta_Y: F(G(Y)) \to Y) is an isomorphism.

  3. A natural transformation (\eta: \text{Id}_C \to F \circ G) such that for every object (X \in C), the component (\eta_X: X \to F(G(X))) is an isomorphism.

  4. A natural transformation (\eta: G \circ F \to \text{Id}_D) such that for every object (Y \in D), the component (\eta_Y: G(F(Y)) \to Y) is an isomorphism.


Correct Option:
Explanation:

The unit of the adjunction is a natural transformation from the identity functor on (C) to the composite functor (G \circ F) that satisfies certain properties, including the component at each object being an isomorphism.

In the context of an adjoint pair of functors (F: C \to D) and (G: D \to C), what is the counit of the adjunction?

  1. A natural transformation (\epsilon: F \circ G \to \text{Id}_D) such that for every object (Y \in D), the component (\epsilon_Y: F(G(Y)) \to Y) is an isomorphism.

  2. A natural transformation (\epsilon: \text{Id}_C \to G \circ F) such that for every object (X \in C), the component (\epsilon_X: X \to G(F(X))) is an isomorphism.

  3. A natural transformation (\epsilon: \text{Id}_D \to F \circ G) such that for every object (Y \in D), the component (\epsilon_Y: Y \to F(G(Y))) is an isomorphism.

  4. A natural transformation (\epsilon: G \circ F \to \text{Id}_C) such that for every object (X \in C), the component (\epsilon_X: G(F(X)) \to X) is an isomorphism.


Correct Option:
Explanation:

The counit of the adjunction is a natural transformation from the composite functor (F \circ G) to the identity functor on (D) that satisfies certain properties, including the component at each object being an isomorphism.

What is the relationship between the unit and counit of an adjoint pair of functors?

  1. The unit and counit are natural transformations that are inverses of each other.

  2. The unit and counit are natural transformations that are composable.

  3. The unit and counit are natural transformations that are equal to each other.

  4. The unit and counit are natural transformations that are orthogonal to each other.


Correct Option:
Explanation:

The unit and counit of an adjoint pair of functors are natural transformations that are inverses of each other, meaning that their composition in either order results in the identity natural transformation.

Given a category (C), what is a monad on (C)?

  1. A triple ((T, \eta, \mu)) consisting of a functor (T: C \to C), a natural transformation (\eta: \text{Id}_C \to T), and a natural transformation (\mu: T \circ T \to T) such that (\mu \circ \mu = \mu \circ T \circ \eta) and (\mu \circ \eta \circ T = \eta \circ \mu).

  2. A triple ((T, \eta, \mu)) consisting of a functor (T: C \to C), a natural transformation (\eta: T \to \text{Id}_C), and a natural transformation (\mu: T \circ T \to T) such that (\mu \circ \mu = \mu \circ T \circ \eta) and (\mu \circ \eta \circ T = \eta \circ \mu).

  3. A triple ((T, \eta, \mu)) consisting of a functor (T: C \to C), a natural transformation (\eta: \text{Id}_C \to T), and a natural transformation (\mu: T \circ T \to T) such that (\mu \circ \mu = \mu \circ \eta \circ T) and (\mu \circ \eta \circ T = \eta \circ \mu).

  4. A triple ((T, \eta, \mu)) consisting of a functor (T: C \to C), a natural transformation (\eta: T \to \text{Id}_C), and a natural transformation (\mu: T \circ T \to T) such that (\mu \circ \mu = \mu \circ T \circ \eta) and (\mu \circ \eta \circ T = \eta \circ \mu).


Correct Option:
Explanation:

A monad on a category (C) is a triple ((T, \eta, \mu)) consisting of a functor (T: C \to C), a unit natural transformation (\eta: \text{Id}_C \to T), and a multiplication natural transformation (\mu: T \circ T \to T) that satisfy certain coherence conditions.

What is the relationship between monads and adjoint pairs of functors?

  1. Every monad can be represented as an adjoint pair of functors.

  2. Every adjoint pair of functors can be represented as a monad.

  3. Monads and adjoint pairs of functors are unrelated concepts.

  4. Monads and adjoint pairs of functors are equivalent concepts.


Correct Option:
Explanation:

Every monad can be represented as an adjoint pair of functors, where the functor (T) is the left adjoint and the functor (G = T \circ T) is the right adjoint.

What is the Kleisli category associated with a monad ((T, \eta, \mu))?

  1. The category whose objects are objects of (C) and whose morphisms are morphisms of (C) that are compatible with the monad structure.

  2. The category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^m), where (n) and (m) are natural numbers.

  3. The category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^{n+1}), where (n) is a natural number.

  4. The category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^{n-1}), where (n) is a natural number.


Correct Option:
Explanation:

The Kleisli category associated with a monad ((T, \eta, \mu)) is the category whose objects are objects of (C) and whose morphisms are morphisms of (C) that are compatible with the monad structure, meaning that they commute with the unit and multiplication natural transformations.

What is the Eilenberg-Moore category associated with a monad ((T, \eta, \mu))?

  1. The category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^m), where (n) and (m) are natural numbers.

  2. The category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^{n+1}), where (n) is a natural number.

  3. The category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^{n-1}), where (n) is a natural number.

  4. The category whose objects are objects of (C) and whose morphisms are morphisms of (C) that are compatible with the monad structure.


Correct Option:
Explanation:

The Eilenberg-Moore category associated with a monad ((T, \eta, \mu)) is the category whose objects are objects of (C) and whose morphisms are natural transformations between functors of the form (T^n \to T^{n+1}), where (n) is a natural number.

What is the relationship between the Kleisli category and the Eilenberg-Moore category associated with a monad?

  1. The Kleisli category is a subcategory of the Eilenberg-Moore category.

  2. The Eilenberg-Moore category is a subcategory of the Kleisli category.

  3. The Kleisli category and the Eilenberg-Moore category are equivalent categories.

  4. The Kleisli category and the Eilenberg-Moore category are unrelated categories.


Correct Option:
Explanation:

The Kleisli category is a subcategory of the Eilenberg-Moore category, as every morphism in the Kleisli category can be represented as a natural transformation between functors of the form (T^n \to T^{n+1}).

What is a free monad on a functor (F: C \to C)?

  1. A monad ((T, \eta, \mu)) such that (T = F) and (\eta) and (\mu) are the identity natural transformations.

  2. A monad ((T, \eta, \mu)) such that (T = F) and (\eta) and (\mu) are natural transformations that satisfy the monad laws.

  3. A monad ((T, \eta, \mu)) such that (T = F \circ F) and (\eta) and (\mu) are natural transformations that satisfy the monad laws.

  4. A monad ((T, \eta, \mu)) such that (T = F \circ F) and (\eta) and (\mu) are the identity natural transformations.


Correct Option:
Explanation:

A free monad on a functor (F: C \to C) is a monad ((T, \eta, \mu)) such that (T = F) and (\eta) and (\mu) are natural transformations that satisfy the monad laws.

What is the relationship between free monads and adjoint pairs of functors?

  1. Every free monad can be represented as an adjoint pair of functors.

  2. Every adjoint pair of functors can be represented as a free monad.

  3. Free monads and adjoint pairs of functors are unrelated concepts.

  4. Free monads and adjoint pairs of functors are equivalent concepts.


Correct Option:
Explanation:

Every free monad can be represented as an adjoint pair of functors, where the functor (T) is the left adjoint and the functor (G = T \circ T) is the right adjoint.

What is a monadic functor?

  1. A functor (F: C \to D) such that there exists a monad ((T, \eta, \mu)) on (C) and a natural transformation (\phi: F \to T) that satisfies certain properties.

  2. A functor (F: C \to D) such that there exists a monad ((T, \eta, \mu)) on (D) and a natural transformation (\phi: F \to T) that satisfies certain properties.

  3. A functor (F: C \to D) such that there exists a monad ((T, \eta, \mu)) on (C) and a natural transformation (\phi: T \to F) that satisfies certain properties.

  4. A functor (F: C \to D) such that there exists a monad ((T, \eta, \mu)) on (D) and a natural transformation (\phi: T \to F) that satisfies certain properties.


Correct Option:
Explanation:

A monadic functor is a functor (F: C \to D) such that there exists a monad ((T, \eta, \mu)) on (C) and a natural transformation (\phi: F \to T) that satisfies certain properties, including the preservation of the unit and multiplication natural transformations.

What is the relationship between monadic functors and adjoint pairs of functors?

  1. Every monadic functor can be represented as an adjoint pair of functors.

  2. Every adjoint pair of functors can be represented as a monadic functor.

  3. Monadic functors and adjoint pairs of functors are unrelated concepts.

  4. Monadic functors and adjoint pairs of functors are equivalent concepts.


Correct Option:
Explanation:

Every monadic functor can be represented as an adjoint pair of functors, where the functor (T) is the left adjoint and the functor (G = T \circ T) is the right adjoint.

What is a Kleisli triple?

  1. A triple ((K, \eta, \mu)) consisting of a functor (K: C \to C), a natural transformation (\eta: \text{Id}_C \to K), and a natural transformation (\mu: K \circ K \to K) such that (\mu \circ \mu = \mu \circ K \circ \eta) and (\mu \circ \eta \circ K = \eta \circ \mu).

  2. A triple ((K, \eta, \mu)) consisting of a functor (K: C \to C), a natural transformation (\eta: K \to \text{Id}_C), and a natural transformation (\mu: K \circ K \to K) such that (\mu \circ \mu = \mu \circ K \circ \eta) and (\mu \circ \eta \circ K = \eta \circ \mu).

  3. A triple ((K, \eta, \mu)) consisting of a functor (K: C \to C), a natural transformation (\eta: \text{Id}_C \to K), and a natural transformation (\mu: K \circ K \to K) such that (\mu \circ \mu = \mu \circ K \circ \eta) and (\mu \circ \eta \circ K = \eta \circ \mu).

  4. A triple ((K, \eta, \mu)) consisting of a functor (K: C \to C), a natural transformation (\eta: K \to \text{Id}_C), and a natural transformation (\mu: K \circ K \to K) such that (\mu \circ \mu = \mu \circ K \circ \eta) and (\mu \circ \eta \circ K = \eta \circ \mu).


Correct Option:
Explanation:

A Kleisli triple is a triple ((K, \eta, \mu)) consisting of a functor (K: C \to C), a unit natural transformation (\eta: \text{Id}_C \to K), and a multiplication natural transformation (\mu: K \circ K \to K) that satisfy certain coherence conditions.

What is the relationship between Kleisli triples and monads?

  1. Every Kleisli triple can be represented as a monad.

  2. Every monad can be represented as a Kleisli triple.

  3. Kleisli triples and monads are unrelated concepts.

  4. Kleisli triples and monads are equivalent concepts.


Correct Option:
Explanation:

Every monad can be represented as a Kleisli triple, where the functor (K = T), the unit natural transformation (\eta) is the same as the unit natural transformation of the monad, and the multiplication natural transformation (\mu) is the composition of the multiplication natural transformation of the monad with itself.

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