In mathematics, the circle group, denoted by , is the multiplicative group of all complex numbers with absolute value 1, that is, the unit circle in the complex plane or simply the unit complex numbers
The circle group forms a subgroup of , the multiplicative group of all nonzero complex numbers. Since is abelian, it follows that is as well. The circle group is also the group of 1×1 complex-valued 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 notation for the circle group stems from the fact that, with the standard topology (see below), the circle group is a 1-torus. More generally (the direct product of with itself times) is geometrically an -torus.
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 , 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 ).
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 , the answer should be 2.155, but we throw away the leading 2, so the answer (in the circle group) is just 0.155 .
The circle group is more than just an abstract algebraic object. It has a natural topology when regarded as a subspace of the complex plane. Since multiplication and inversion are continuous functions on , 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 (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 dimensional compact, connected, abelian Lie group is isomorphic to .
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 the 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 , the first unitary group.
The last equality is Euler's formula or the complex exponential. The real number ? corresponds to the angle (in radians) on the unit circle as measured counterclockwise from the positive x axis. That this map is a homomorphism follows from the fact that the multiplication of unit complex numbers corresponds to addition of angles:
This exponential map is clearly a surjective function from to . It is not, however, injective. The kernel of this map is the set of all integer multiples of . By the first isomorphism theorem we then have that
After rescaling we can also say that is isomorphic to .
This isomorphism has the geometric interpretation that multiplication by a unit complex number is a proper rotation in the complex (and real) plane, and every such rotation is of this form.
Every compact Lie group 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 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
must take values in . Therefore, the irreducible representations of the circle group are just the homomorphisms from the circle group to itself.
These representations are all inequivalent. The representation is conjugate to ,
taking values in . Here we only have positive integers since the representation is equivalent to .
The circle group is a divisible group. Its torsion subgroup is given by the set of all th roots of unity for all , and is isomorphic to . The structure theorem for divisible groups and the axiom of choice together tell us that is isomorphic to the direct sum of with a number of copies of .
The number of copies of must be (the cardinality of the continuum) in order for the cardinality of the direct sum to be correct. But the direct sum of copies of is isomorphic to , as is a vector space of dimension over . Thus
can be proved in the same way, since is also a divisible abelian group whose torsion subgroup is the same as the torsion subgroup of .