Category Theory Cheat Post

After learning Haskell I finally decided to jump the train and start learning Category Theory with the help of the book Conceptual Mathematics – A first introduction to categories. I greatly encourage you to do the same, especially if you are a self-taught programmer.

The following is a simple cheat sheet that is meant to help me (and maybe you?) remember all those new words and constructs. Please poke me if you find any errors or have an improvement suggestion.

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Basics

Associative

The order of operator application does not matter.

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Commutative

The order of the operands does not matter.

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Idempotent [idem + potence = (same + power)]

The endomorphism is idempotent under composition if

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Involution

The automorphism is an involution of A if

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Category

What makes a Category?
– Objects
– Morphisms (maps)
– For each object in  an identity map   must exist.
– For each pair of maps  and  a composite map  must exist.

– Composition  must follow the identity and associative laws:

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Morphisms

Monomorphism (injective) [monos = one]

– distinctive
It maps distinct objects onto distinct objects. The codomain is at least as large as the domain.

– left cancellative
with

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Split Monomorphism

A monomorphism which has a left inverse for which , i.e. a retraction.

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Epimorphism (surjective) [epis = against]

– covering
All objects of the codomain are hit at least once. The domain is at least as large as the codomain.

– right cancellative
with

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Split Epimorphism

An epimorphism which has a right inverse for which , i.e. a section.

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Isomorphism (bijective) [isos = equal]

– is a monomorphism and epimorphism
– domain and codomain have the ‘same size’
– distinctive
– invertible
 with and 

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Automorphism [auto = self]

– is an isomorphism
– also called permutation

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Homomorphism [homos = same]

– structure preserving map
– if and are groups, then f is a homomorphism if

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Endomorphism [endos = inside]

– is a homomorphism

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Constant map

A map is called constant if all elements of A are send onto the same element of B.

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Division of maps

Determination or extension

If it has a solution g, we say h is ‘determined by’ f or h ‘depends only on’ f.

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Retraction

Special case of the determination problem in which and
– is an epimorphism
– left inverse of

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Choice or lifting

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Section

Special case of the choice problem in which and
– is a monomorphism
– right inverse of

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