Generally, a theta function (-function, -function) is a holomorphic section of a (principally polarizing) holomorphic line bundle over a complex torus / abelian variety. (e.g. Polishchuk 03, section 17) and in particular over a Jacobian variety (Beauville) such as prequantum line bundles for (abelian) gauge theory. The line bundle being principally polarizing means that its space of holomorphic sections is 1-dimensional, hence that it determines the -function up to a global complex scale factor. Typically these line bundles themselves are Theta characteristics. Expressed in coordinates on the covering of the complex torus , a -function appears as an actual function satisfying certain transformation properties, and this is how theta functions are considered.
Those theta functions encoding line bundles on a Jacobian variety of a Riemann surface typically vary in a controlled way with the complex structure modulus of and are hence really functions also of this variable with certain transformation properties. These are the Riemann theta functions. If here is complex 1-dimensional of genus (hence a complex elliptic curve) then such a function of two variables with the pertinent transformation properties is properly called a Jacobi form. Nevertheless the principal examples here are called Jacobi theta functions.
Specifically in the context of number theory/arithmetic geometry, by the theta function one usually means the Jacobi theta function (see there for more) for . While this is the historically first and archetypical function from which all modern generalizations derive their name, notice that at fixed as a function in the “theta function” is not actually a section of a line bundle anymore.
|context/function field analogy||theta function||zeta function (= Mellin transform of )||eta function and L-function of Galois representation/flat connection||special values of L-functions|
|physics/2d CFT||partition function as function of complex structure of worldsheet (hence polarization of phase space) and background gauge field/source||analytically continued Feynman propagator||analytically continued Dirac propagator||regularized Feynman propagator / regularized Dirac propagator / path integral|
|analysis||zeta function of an elliptic differential operator||eta function of a self-adjoint operator||functional determinant|
|complex analytic geometry||section of line bundle over Jacobian variety in terms of covering coordinates on||zeta function of a Riemann surface||Selberg zeta function|
|arithmetic geometry for a function field||Goss zeta function (for arithmetic curves) and Weil zeta function (in higher dimensional arithmetic geometry)|
|arithmetic geometry for a number field||Hecke theta function, automorphic form||Dedekind zeta function (being the Artin L-function for the trivial representation)||Artin L-function of a Galois representation , expressible “in coordinates” (by Artin reciprocity) as a finite-order Hecke L-function (for 1-dimensional representations) and generally (via Langlands correspondence) by an automorphic L-function (for higher dimensional reps)||class number regulator|
|arithmetic geometry for||Jacobi theta function||Riemann zeta function||Artin L-function of a Galois representation , expressible “in coordinates” (via Artin reciprocity) as a Dirichlet L-function (for 1-dimensional Galois representations) and generally (via Langlands correspondence) as an automorphic L-function|
Theta functions are naturally thought of as being the states in the geometric quantization of the given complex space, the given holomorphic line bundle being the prequantum line bundle and the condition of holomorphicity of the section being the polarization condition. See for instance (Tyurin 02). In this context they play a proming role specifically in the quantization of higher dimensional Chern-Simons theory and of self-dual higher gauge theory. See there for more.
Specifically the fact that in geometric quantization of Chern-Simons theory in the abelian case, and the holographically dual partition functions of the WZW model the choice of polarization is induced from the choice of complex structure on a given Riemann surface and for each such choice there is then a section/partition function depending on a coordinte in the Jacobian is reflected in the double coordinate dependence of the theta function:
Say that a system of multipliers is a system of invertible holomorphic functions
satisfying the cocycle condition
Then a theta function is a holomorphic function
for which there is a system of multipliers satisfying the functional equation which says that for each and we have
|line bundle||square root||choice corresponds to|
|canonical bundle||Theta characteristic||over Riemann surface and Hermitian manifold (e.g.Kähler manifold): spin structure|
|density bundle||half-density bundle|
|canonical bundle of Lagrangian submanifold||metalinear structure||metaplectic correction|
|determinant line bundle||Pfaffian line bundle|
|quadratic secondary intersection pairing||partition function of self-dual higher gauge theory||integral Wu structure|
Introductions to the traditional notion include
D.H. Bailey et al, The Miracle of Theta Functions (web)
M. Bertola, Riemann surfaces and theta functions (pdf)
Modern textbook accounts include
David Mumford, Tata Lectures on Theta, Birkhäuser 1983
Further discussion with an emphasis of the origin of theta functions in geometric quantization is in
Yuichi Nohara, Independence of polarization in geometric quantization (pdf)
Luis Alvarez-Gaumé, Jean-Benoit Bost, Gregory Moore, Philip Nelson, Cumrun Vafa, Bosonization on higher genus Riemann surfaces, Communications in Mathematical Physics, Volume 112, Number 3 (1987), 503-552 (Euclid)
and more generally the partition functions of connection-twisted Dirac operators on even-dimensional locally symmetric spaces is discussed in
The generalization of theta functions to automorphic forms is due to
see Gelbhart 84, page 35 (211) for review.