Fun Stuff · Mathematics

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.



The order of operator application does not matter.
a * (b * c) = (a * b) *c



The order of the operands does not matter.
a * b = b * a


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

The endomorphism A\xrightarrow{e}A is idempotent under composition if e \circ e = e



The automorphism A\xrightarrow{\theta}A is an involution of A if \theta \circ \theta = 1_A



What makes a Category?
– Objects
– Morphisms (maps)
– For each object in A an identity map 1_A  must exist.
– For each pair of maps A\xrightarrow{f}B and B\xrightarrow{g}C a composite map A\xrightarrow{g \circ f}C must exist.

– Composition \circ must follow the identity and associative laws:

1_B\circ f = f

f\circ 1_A = f

a\circ (b\circ c) = (a\circ b)\circ c



Monomorphism (injective) [monos = one]

– distinctive
It maps distinct objects onto distinct objects. The codomain is at least as large as the domain.
x \not= y\implies f(x) \not= f(y)

– left cancellative
f\circ g_1 = f\circ g_2\implies g_1=g_2 with Z\xrightarrow{g_1, g_2} A\xrightarrow{f} B


Split Monomorphism

A monomorphism A\xrightarrow{f}B which has a left inverse B\xrightarrow{r}A for which r\circ f = 1_A, i.e. a retraction.
A\xrightarrow{f}B\xrightarrow{r} A


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.
\forall b\in B\exists a\in A (f(a)=b)

– right cancellative
h_1\circ f = h_2\circ f\implies h_1=h_2 with A\xrightarrow{f} B\xrightarrow{h_1, h_2} C


Split Epimorphism

An epimorphism A\xrightarrow{f}B which has a right inverse B\xrightarrow{s} A for which f\circ s = 1_B, i.e. a section.


Isomorphism (bijective) [isos = equal]

– is a monomorphism and epimorphism
A\cong B domain and codomain have the ‘same size’
– distinctive
– invertible
A\xrightarrow{f}B\xrightarrow{f^{-1}}A with f\circ f^{-1} = 1_B and f^{-1}\circ f = 1_A


Automorphism [auto = self]

– is an isomorphism A\xrightarrow{f}A
– also called permutation


Homomorphism [homos = same]

– structure preserving map
– if (G, *) and (H, *') are groups, then f is a homomorphism if f(g * h) = f(g) *' f(h)


Endomorphism [endos = inside]

– is a homomorphism A\xrightarrow{f}A


Constant map

\exists b\in B \forall a \in A(f(a) = b)
A map A\xrightarrow{f}B is called constant if all elements of A are send onto the same element of B.


Division of maps

Determination or extension

A\xrightarrow{f}B\xrightarrow{g?}C\xleftarrow{h = g\circ f}A
If it has a solution g, we say h is ‘determined by’ f or h ‘depends only on’ f.



Special case of the determination problem A\xrightarrow{f}B\xrightarrow{r?}A\xleftarrow{1_A}A in which h = 1_A and r\circ f = 1_A
– is an epimorphism
– left inverse of f


Choice or lifting

A\xrightarrow{f?}B\xrightarrow{g}C\xleftarrow{h = g\circ f}A



Special case of the choice problem B\xrightarrow{s?}A\xrightarrow{f}B\xleftarrow{1_B}B in which h = 1_B and f\circ s = 1_B
– is a monomorphism
– right inverse of f



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