3. Familiar Group
โข The integers under addition, โค, + .
โข The real numbers under addition, โ , + .
โข The complex numbers under addition, โ , + .
โข The integers modulo n under addition,
โค๐ , + .
โข The rationals under addition, โ , + .
In each case, associativity is clear, the identity is 0,
and the inverse of ๐ฅ is โ๐ฅ (in โค๐, โ๐ฅ = ๐ โ ๐ฅ).
Example : 1
4. โข The nonzero rationalsunder
multiplication, โโ
,ร .
โข The nonzero real numbers under
multiplication, โโ
,ร .
โข The nonzero complex numbers under
multiplication, โโ
,ร .
In each case, associativity is clear, the identity is 1,
and the inverse of ๐ฅ is 1/๐ฅ.
Familiar Group
Example : 2
5. An example of a non-abelian group is the group of
all ๐ ร ๐ invertible matrices under matrix
multiplication:
๐๐ = ๐ด =
๐1,1 โฏ ๐1,๐
โฎ โฑ โฎ
๐๐,1 โฏ ๐๐,๐
๐๐,๐ โ ๐ , det ๐ด โ 0 .
In each of the examples above, the binary operations
(+ or ร) are commutative (that is, ๐ + ๐ = ๐ + ๐
and ๐ ร ๐ = ๐ ร ๐). A group in which the binary
operation is commutative is called an abelian group.
Familiar Group
Example : 3
6. It is not the case (in general) that
๐ข ร ๐ฃ ร ๐ค = ๐ข ร ๐ฃ ร ๐ค.
For example, ๐ ร ๐ ร ๐ = ๐ ร ๐ = โ๐ but
๐ ร ๐ ร ๐ = 0 ร ๐ = 0.
A Strange Example
In the above examples, associativity is โclear.โ In fact, it is
rather rare to encounter a binary operation that is not
associative. One such example, which you see in Linear
Algebra and Calculus 3 is the cross product of vectors in โ3
.
So we cannot form a group using vectors and the
cross product!
7. Each of the above examples of groups are
โalgebraic.โ We now introduce groups based on
geometric properties.
Definition. An isometryof n-dimensional space โ๐
is a function from โ๐
onto โ๐
that preserves
distance. [Gallian, page 461]
Geometry and Isometrics
More precisely, ๐ is an isometry from โ๐
to โ๐
if
for all ๐ฅ, ๐ฆ โ โ๐
we have
๐ ๐ฅ, ๐ฆ = ๐ ๐ ๐ฅ , ๐ ๐ฆ
where ๐ is a metric on โ๐
.
Symmetry Groups
8. Definition. Let ๐น be a set of points in โ๐
.
The symmetry group of ๐น in โ๐
is the set of
all isometries of โ๐
that carry ๐น onto itself.
The group operation is function composition.
[Gallian, page 461]
Symmetry Groups
9. Symmetry Group
Example. Consider the line segment I in โfrom
๐ = 0 to ๐ = 1. There are two isometries which
map I onto itself: ๐0 ๐ฅ = ๐ฅ and ๐1 ๐ฅ = 1 โ ๐ฅ.
1
0 1
0
1
0 1
0
The โmultiplication tableโ
for the symmetry group
with elements ๐0 and ๐1 is:
๐0 ๐1
๐0 ๐0 ๐1
๐1 ๐1 ๐0
ยฐ
๐0 ๐1
10. Example. Consider the line segment I in โ2from ๐ = 0
to ๐ = 1. There are four isometries which map I onto
itself:
1. ๐0 ๐ฅ, ๐ฆ = (๐ฅ, ๐ฆ), the identity,
2. ๐1 ๐ฅ, ๐ฆ = (1 โ ๐ฅ, ๐ฆ), reflection about the line ๐ฅ =
1/2,
3. ๐2 ๐ฅ, ๐ฆ = (๐ฅ, โ๐ฆ), reflection about the line ๐ฆ = 0,
and
4. ๐3 ๐ฅ, ๐ฆ = (1 โ ๐ฅ, โ๐ฆ), a combination of ๐1 and ๐2.
Symmetry Group
11. x
y
x
y
๐0
๐1
๐2
๐3
Identity
Reflection about
๐ฆ = 0
Reflection about
๐ฅ = 1/2
Reflection about
๐ฅ = 1/2 and ๐ฆ = 0
Symmetry Group
๐0 ๐1 ๐2 ๐3
๐0 ๐0 ๐1 ๐2 ๐3
๐1 ๐1 ๐0 ๐3 ๐2
๐2 ๐2 ๐3 ๐0 ๐1
๐3 ๐3 ๐2 ๐1 ๐0
The multiplication
table for this group is
given here. The group
is denoted ๐ท2, .
ยฐ
ยฐ
16. We find that the multiplication table for the group
of symmetries of an equilateral triangle is:
โ ๐0 ๐1 ๐2 ๐1 ๐2 ๐3
๐0 ๐0 ๐1 ๐2 ๐1 ๐2 ๐3
๐1 ๐1 ๐2 ๐0 ๐3 ๐1 ๐2
๐2 ๐2 ๐0 ๐1 ๐2 ๐3 ๐1
๐1 ๐1 ๐2 ๐3 ๐0 ๐1 ๐2
๐2 ๐2 ๐3 ๐1 ๐2 ๐0 ๐1
๐3 ๐3 ๐1 ๐2 ๐1 ๐2 ๐0
This group of symmetries is the dihedral group ๐ท3.
Notice that it contains a subgroup or order 3.
Symmetry Group
17. Dihedral Groups
The symmetries of a regular ๐-gon form the dihedral
group, ๐ท๐, , which consists of 2๐ permutations.
These groups are generated by the two fundamental
permutations: rotations and reflections.
ยฐ
18. Cyclic Groups
If we consider the symmetries of a regular ๐-gon which
only consist of the rotations (and not the reflections) then
we get a subgroup of the dihedral group๐ท๐ which consists
of the ๐ rotational permutations.
This group of ๐ rotational permutations forms the cyclic
group of order ๐. The cyclic group of order ๐ is the same as
(i.e., โisomorphic toโ) the integers modulo ๐, โค๐ , + .
Since, for each ๐, the cyclic group of order ๐ is a subgroup
of the dihedral group of order 2๐, we often drop the binary
operation and write this as: โค๐ < ๐ท๐.
20. Generators of a Group
The cyclic group โค๐ can be โgeneratedโ by the elementary
rotation ๐1. That is, each element of โค๐ is a power of ๐1:
๐2 = ๐1 โ ๐1, ๐3 = ๐1 โ ๐1 โ ๐1, etc. In fact, the formal
definition of a cyclic group is a group which is generated
by a single element.
The dihedral group ๐ท๐ can be generated by two
symmetries: a rotation and a reflection. This is the reason
these groups are called dihedral groups.
21. Finite Symmetry Groups
Definition. A group ๐บ,โ is finite if the set ๐บ has a finite
number of elements: ๐บ < โ.
Theorem. The only finite symmetry groups of a set of
points in โ2 (that is, the only โplane symmetry groupsโ or
โgroups of isometries of the planeโ) are the groups โค๐ and
๐ท๐ for some ๐. These groups are sometimes called rosette
groups. [Fraleigh, page 115; Gallian, page 463]
We now explore infinite plane symmetry groups. This topic
is covered (briefly) in Fraleigh in Section II.12 (โPlane
Isometriesโ) and in some detail in Gallianโs Chapter 28
(โFrieze Groups and Crystallographic Groupsโ).