In mathematics, the theta representation is a particular representation of the Heisenberg group of quantum mechanics. It gains its name from the fact that the Jacobi theta function is invariant under the action of a discrete subgroup of the Heisenberg group. The representation was popularized by David Mumford.
The theta representation is a representation of the continuous Heisenberg group over the field of the real numbers. In this representation, the group elements act on a particular Hilbert space. The construction below proceeds first by defining operators that correspond to the Heisenberg group generators. Next, the Hilbert space on which these act is defined, followed by a demonstration of the isomorphism to the usual representations.
Let f(z) be a holomorphic function, let a and b be real numbers, and let be fixed, but arbitrary complex number in the upper half-plane; that is, so that the imaginary part of is positive. Define the operators Sa and Tb such that they act on holomorphic functions as
It can be seen that each operator generates a one-parameter subgroup:
However, S and T do not commute:
A general group element then acts on a holomorphic function f(z) as
Here, is the imaginary part of and the domain of integration is the entire complex plane. Let be the set of entire functions f with finite norm. The subscript is used only to indicate that the space depends on the choice of parameter . This forms a Hilbert space. The action of given above is unitary on , that is, preserves the norm on this space. Finally, the action of on is irreducible.
The above theta representation of the Heisenberg group is isomorphic to the canonical Weyl representation of the Heisenberg group. In particular, this implies that and L2(R) are isomorphic as H-modules. Let
stand for a general group element of . In the canonical Weyl representation, for every real number h, there is a representation acting on L2(R) as
for and .
Define the subgroup as
The Jacobi theta function is defined as
when a and b are integers. It can be shown that the Jacobi theta is the unique such function.
- David Mumford, Tata Lectures on Theta I (1983), Birkhauser, Boston ISBN 3-7643-3109-7