CIRCLE GROUP
From Wikipedia, the free encyclopedia
In
mathematics, the
circle group, denoted by
T, is the multiplicative
group of all
complex numbers with absolute value 1, i.e., the
unit circle in the complex plane.
The circle group forms a
subgroup of
C×, the multiplicative group of all nonzero complex numbers. Since
C× is
abelian, it follows that
T is as well. The circle group is also the group
U(1) of 1×1
unitary matrices; these act on the complex plane by rotation about the origin. The circle group can be parametrized by the angle θ of rotation by
This is the
exponential map for the circle group.
The circle group plays a central role in
Pontryagin duality, and in the theory of
Lie groups.
The notation
T for the circle group stems from the fact that
Tn (the
direct product of
T with itself
n times) is geometrically an
n-
torus. The circle group is then a 1-torus.
Elementary introduction
Multiplication on the circle group is equivalent to addition of angles
One way to think about the circle group is that it describes how to add
angles, where only angles between 0° and 360° are permitted. For example, the diagram illustrates how to add 150° to 270°. The answer should be 150° + 270° = 420°, but when thinking in terms of the circle group, we need to "forget" the fact that we have wrapped once around the circle. Therefore we adjust our answer by 360° which gives 420° − 360° = 60°.
Another description is in terms of ordinary addition, where only numbers between 0 and 1 are allowed (with 1 corresponding to a full rotation). To achieve this, we might need to throw away digits occurring before the decimal point. For example, when we work out 0.784 + 0.925 + 0.446, the answer should be 2.155, but we throw away the leading 2, so the answer (in the circle group) is just 0.155.
Topological and analytic structure
The circle group is more than just an abstract algebraic group. It has a natural topology when regarded as a
subspace of the complex plane. Since multiplication and inversion are
continuous functions on
C×, the circle group has the structure of a
topological group. Moreover, since the unit circle is a
closed subset of the complex plane, the circle group is a closed subgroup of
C× (itself regarded as a topological group).
One can say even more. The circle is a 1-dimensional real
manifold and multiplication and inversion are
real-analytic maps on the circle. This gives the circle group the structure of a
one-parameter group, an instance of a
Lie group. In fact,
up to isomorphism, it is the unique 1-dimensional
compact,
connected Lie group. Moreover, every
n-dimensional compact, connected, abelian Lie group is isomorphic to
Tn.
Isomorphisms
The circle group shows up in a variety of forms in mathematics. We list some of the more common forms here. Specifically, we show that
Note that
slash denotes here
quotient group.
The set of all 1×1
unitary matrices clearly coincides with the circle group; the unitary condition is equivalent to the condition that its element have absolute value 1. Therefore, the circle group is canonically isomorphic to U(1), the first
unitary group.
The
exponential function gives rise to a
group homomorphism exp :
R →
T from the additive real numbers
R to the circle group
T via the map
The last equality is
Euler's formula. The real number θ corresponds to the angle on the unit circle as measured from the positive
x-axis. That this map is a homomorphism follows from the fact the multiplication of unit complex numbers corresponds to addition of angles:
This exponential map is clearly a
surjective function from
R to
T. It is not, however,
injective. The
kernel of this map is the set of all
integer multiples of 2π. By the
first isomorphism theorem we then have that
After rescaling we can also say that
T is isomorphic to
R/
Z.
If complex numbers are realized as 2×2 real
matrices (see
complex number), the unit complex numbers correspond to 2×2
orthogonal matrices with unit
determinant. Specifically, we have
The circle group is therefore isomorphic to the
special orthogonal group SO(2). This has the geometric interpretation that multiplication by a unit complex number is a proper rotation in the complex plane, and every such rotation is of this form.
Properties
Any compact Lie group
G of dimension > 0 has a
subgroup isomorphic to the circle group. That means that, thinking in terms of
symmetry, a compact symmetry group acting
continuously can be expected to have one-parameter circle subgroups acting; the consequences in physical systems are seen for example at
rotational invariance, and
spontaneous symmetry breaking.
The circle group has many
subgroups, but its only proper
closed subgroups consist of
roots of unity: For each integer
n > 0, the
nth roots of unity form a
cyclic group of order
n, which is unique up to isomorphism.
Representations
The
representations of the circle group are easy to describe. It follows from
Schur's lemma that the
irreducible complex representations of an abelian group are all 1-dimensional. Since the circle group is compact, any representation ρ :
T →
GL(1,
C) ≅
C×, must take values in
U(1)≅
T. Therefore, the irreducible representations of the circle group are just the homomorphisms from the circle group to itself. Every such homomorphism is of the form
These representations are all inequivalent. The representation φ
-n is
conjugate to φ
n,
These representations are just the
characters of the circle group. The
character group of
T is clearly an
infinite cyclic group generated by φ
1:
The irreducible
real representations of the circle group are the
trivial representation (which is 1-dimensional) and the representations
taking values in SO(2). Here we only have positive integers
n since the representation
ρ − n is equivalent to
ρn.
Group structure
In this section we will forget about the topological structure of the circle group and look only at its structure as an abstract group.
The circle group
T is a
divisible group. Its
torsion subgroup is given by the set of all
nth
roots of unity for all
n, and is isomorphic to
Q/
Z. The structure theorem for divisible groups tells us that
T is isomorphic to the
direct sum of
Q/
Z with a number of copies of
Q. The number of copies of
Q must be
c (the
cardinality of the continuum) in order for the cardinality of the direct sum to be correct. But the direct sum of
c copies of
Q is isomorphic to
R, as
R is a
vector space of dimension
c over
Q. Thus
The isomorphism
can be proved in the same way, as
C× is also a divisible abelian group whose torsion subgroup is the same as the torsion subgroup of
T.
See also
References
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