Maths - Maps

Functions (mappings)

A function (or mapping) φ from a set 'A' to a set 'B' is a rule that assigns to each element 'a' of A' exactly one element 'b' of B. This is written:

φ: A → B

Domain and Codomain

Domain

This is the set which is the source of the function.

domain

Codomain

This is the set which is the destination of the function.

codomain

Partial Functions

On this page we will mostly assume that every element in the domain maps to an element in the domain, however this is not the case for partial functions.

Examples of partial functions, in the real numbers, are:

This is also interesting in computing where a function may represent a computation. Where this computation is very complicated it may not be possible to be sure that every input to the function will produce a valid result.

The mathematics of partial functions is discussed further on the page here.

Injective, Surjective and Bijective Functions

Injective (one-to-one function)

Every element in the domain is mapped to a unique element in the codomain. There may be elements in the domain that are not used. (size of domain <= size of codomain)

injective function

Surjective (onto)

Elements in the domain may be mapped to the same element in the codomain but every element in the codomain is targeted. (size of domain >= size of codomain)

surjective function

Bijective function
(one-to-one correspondence)
(Injective and Surjective)

Every element in the domain is mapped to a unique element in the codomain and every element in the codomain is mapped to a unique element in the domain. (size of domain = size of codomain)

bijective function

Composition of Functions

If we apply a function 'g' to and element x and then apply function h' to the result we get a combined function: (g o h)(x)

where:

In other words we have:

(g o h)(x) = h(g(x))

composition of functions

Function composition is always associative

(f o g)o h = f o (g o h)

Morphisms

homomorphism

A morphism is a function f between sets with structure (such as group, ring or fields) that preserve some element of the structure.

For instance :

f(g * h) = f(g) * f(h)

Types of Morphisms

morphism
homomorphism
general form, same meaning as homomorphism.
isomorphism a morphism where the inverse f-1 is also a morphism. f-1(g * h) = f-1(g) * f-1(h)
automorphism a isomorphism onto itself
endomorphism a morphism onto itself

Examples

Morphisms between Lattices on page here.

Image and Kernel

See short exact sequence.

Image image of morphism

Kernel

The set of all elements 'x' of a structure 'X' such that f(x) is the identity element of structure Y.

kernel

Coimage

the coimage of a morphism f: A → B is the quotient:

coim f = A/ker(f)

coimage

Cokernel

the cokernel of a linear mapping of vector spaces f :
A → B is the quotient:

cokernel = B/im(f)

cokernel

Orbit and Stabiliser

see Sylow theory

Orbit

orb(s) - When a group G acts on a set S, the orbit of any s in S is the set of elements of S that G arrows can reach from s.

orb(s) = {φ(s) | φ∈G}

orb(s)

Stabiliser

stab(s) - The stabilizer of an element s in S is the set of group elements g that don't move s. A configuration s in S is called stable if no actions move s.

stab(s) = {φ∈G | φ(s)=s}

stab(s)

Types of Map

notation example meaning
AmapB   Map / Morphism / Function between mathematical structures that relates or preserves the structures in some way. May be injection, surjection or bijection (injection can be specifically indicated see below)
a mapElement b   A function in terms of elements of the structures a∈A, b∈B
map dot   A function that is valid if all the functions drawn as solid lines are valid
map injective   injective morphism (embedding)
map canonical   canonical map - map of G onto factor group G/H where H is a normal subgroup of G

More notation on this page.

More about category theory on this page.


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see also:

 

Correspondence about this page

Book Shop - Further reading.

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Terminology and Notation

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