Maths - Concrete Category - Sets

In this category theory section we will use a slightly different notion of a set than we used when we were discussing sets, in their own right, on this page.

One definition of concrete categorories is a set together with some form of 'structure' (by structure we mean functions, mappings and operations within the category). So we can think of sets as being an elemental category without any such structure. Or we could start with some other type of concrete category and apply a 'forgetful functor' to remove the structure.

So, in this context, we are thinking of a set as a 'bag of points' unrelated to each other in any way.

Special Sets

The Empty Set

This has no elements or points. This is usually denoted 0, Ø or {}.

The One Element (singleton) Set

This has exacly one element (or point). Since category theory tends to compare objects 'up to isomorpism' then, from this point of view, we can't distingush between one-element-sets so they are effectivly the same as each other. This is usually denoted 1.

Universal Properties of Set

See this page for a discussion of universal properties.

Initial Terminal

Ø = {}

empty set

{1}or {a} ...

one element set

initial arrow category terminal arrow category


product arrow category sum arrow category

cartesian product


disjoint union



Setop has the same diagrams as set but with the arrows reversed.

So how do we reverse any map betweens sets? We need something completely equivilant to the reverse not just some unique approximation like adjunctions.

Bijective maps are easy, we just reverse the arrows, but we also want to reverse injective and surjective maps:

Injective (one-to-one function)

In order to reverse an injective function we can reverse all the arrows but we need somewhere for those elements, without arrows, to be mapped back to. A first thought is the empty set. Here we are mapping each element in 'A' to a set (so we have sets within sets), for consistancy we also wrap the other elements in their own set.

injective function
reverse injective

Surjective (onto)

In order to reverse a surjective function we can again map into a 'set of sets'. This time we replace two arrows from a1 and a3 with a single arrow from a set containing a1 and a3. This single arrow can now be reversed.

surjective function

reverse surjective

So this sugests that the opposite of set could be a set of sets. This complete set of all posible sets of set is known as a powerset.

  powerset 1

A poweset is sometimes denoted P(Set).

Alternative notations are the exponential function:



set -> 2

powerset 2

Functors in Setop

Functor (contravarient) between powersets.

In the diagram on the right: element 'a' is mapped to 'a' . Elements 'b' and 'c' are both mapped to 'b'. The diagram shows corresponding powesets.

This diagram does not commute but it does send 'x' to a set containing 'x'.

powerset functor

Universal Properties in Setop

Initial Terminal


set containing the empty set and the complete set


set containing the empty set

powerset initial final

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Correspondence about this page

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cover Modern Graph Theory (Graduate Texts in Mathematics, 184)

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