Theta function

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For other θ functions, see Theta function (disambiguation).
Jacobi's original theta function with and with nome . Conventions are (Mathematica): this is:

In mathematics, theta functions are special functions of several complex variables. They are important in many areas, including the theories of abelian varieties and moduli spaces, and of quadratic forms. They have also been applied to soliton theory. When generalized to a Grassmann algebra, they also appear in quantum field theory.[1]

The most common form of theta function is that occurring in the theory of elliptic functions. With respect to one of the complex variables (conventionally called z), a theta function has a property expressing its behavior with respect to the addition of a period of the associated elliptic functions, making it a quasiperiodic function. In the abstract theory this comes from a line bundle condition of descent.

Jacobi theta function[edit]

Jacobi theta 1
Jacobi theta 2
Jacobi theta 3
Jacobi theta 4

There are several closely related functions called Jacobi theta functions, and many different and incompatible systems of notation for them. One Jacobi theta function (named after Carl Gustav Jacob Jacobi) is a function defined for two complex variables z and τ, where z can be any complex number and τ is confined to the upper half-plane, which means it has positive imaginary part. It is given by the formula

where q = exp(πiτ) and η = exp(2πiz). It is a Jacobi form. If τ is fixed, this becomes a Fourier series for a periodic entire function of z with period 1; in this case, the theta function satisfies the identity

The function also behaves very regularly with respect to its quasi-period τ and satisfies the functional equation

where a and b are integers.

Theta function with different nome . The black dot in the right-hand picture indicates how is changing with .
Theta function with different nome . The black dot in the right-hand picture indicates how is changing with .

Auxiliary functions[edit]

The Jacobi theta function defined above is sometimes considered along with three auxiliary theta functions, in which case it is written with a double 0 subscript:

The auxiliary (or half-period) functions are defined by

This notation follows Riemann and Mumford; Jacobi's original formulation was in terms of the nome rather than τ. In Jacobi's notation the θ-functions are written:

The above definitions of the Jacobi theta functions are by no means unique. See Jacobi theta functions (notational variations) for further discussion.

If we set z = 0 in the above theta functions, we obtain four functions of τ only, defined on the upper half-plane (sometimes called theta constants.) These can be used to define a variety of modular forms, and to parametrize certain curves; in particular, the Jacobi identity is

which is the Fermat curve of degree four.

Jacobi identities[edit]

Jacobi's identities describe how theta functions transform under the modular group, which is generated by τ ↦ τ+1 and τ ↦ −1/τ. Equations for the first transform are easily found since adding one to τ in the exponent has the same effect as adding 1/2 to z (n is congruent to n squared modulo 2). For the second, let

Then

Theta functions in terms of the nome[edit]

Instead of expressing the Theta functions in terms of and , we may express them in terms of arguments and the nome q, where and . In this form, the functions become

We see that the theta functions can also be defined in terms of w and q, without a direct reference to the exponential function. These formulas can, therefore, be used to define the Theta functions over other fields where the exponential function might not be everywhere defined, such as fields of p-adic numbers.

Product representations[edit]

The Jacobi triple product tells us that for complex numbers w and q with |q| < 1 and w ≠ 0 we have

It can be proven by elementary means, as for instance in Hardy and Wright's An Introduction to the Theory of Numbers.

If we express the theta function in terms of the nome and then

We therefore obtain a product formula for the theta function in the form

In terms of w and q:

where is the q-Pochhammer symbol and is the q-theta function. Expanding terms out, the Jacobi triple product can also be written

which we may also write as

This form is valid in general but clearly is of particular interest when z is real. Similar product formulas for the auxiliary theta functions are

Integral representations[edit]

The Jacobi theta functions have the following integral representations:

Explicit values[edit]

See [2]

Some series identities[edit]

The next two series identities were proved by István Mező

[3]

These relations hold for all 0 < q < 1. Specializing the values of q, we have the next parameter free sums

and

Zeros of the Jacobi theta functions[edit]

All zeros of the Jacobi theta functions are simple zeros and are given by the following:

where m, n are arbitrary integers.

Relation to the Riemann zeta function[edit]

The relation

was used by Riemann to prove the functional equation for the Riemann zeta function, by means of the integral

which can be shown to be invariant under substitution of s by 1 − s. The corresponding integral for z not zero is given in the article on the Hurwitz zeta function.

Relation to the Weierstrass elliptic function[edit]

The theta function was used by Jacobi to construct (in a form adapted to easy calculation) his elliptic functions as the quotients of the above four theta functions, and could have been used by him to construct Weierstrass's elliptic functions also, since

where the second derivative is with respect to z and the constant c is defined so that the Laurent expansion of at z = 0 has zero constant term.

Relation to the q-gamma function[edit]

The fourth theta function – and thus the others too – is intimately connected to the Jackson q-gamma function via the relation[4]

Relations to Dedekind eta function[edit]

Let η(τ) be the Dedekind eta function, and the argument of the theta function as the nome . Then,

and,

See also the Weber modular functions.

Elliptic modulus[edit]

The elliptic modulus is

and the complementary elliptic modulus is

A solution to heat equation[edit]

The Jacobi theta function is the fundamental solution of the one-dimensional heat equation with spatially periodic boundary conditions. Taking z = x to be real and τ = it with t real and positive, we can write

which solves the heat equation

This theta-function solution is 1-periodic in x, and as t → 0 it approaches the periodic delta function, or Dirac comb, in the sense of distributions

.

General solutions of the spatially periodic initial value problem for the heat equation may be obtained by convolving the initial data at t = 0 with the theta function.

Relation to the Heisenberg group[edit]

The Jacobi theta function is invariant under the action of a discrete subgroup of the Heisenberg group. This invariance is presented in the article on the theta representation of the Heisenberg group.

Generalizations[edit]

If F is a quadratic form in n variables, then the theta function associated with F is

with the sum extending over the lattice of integers Zn. This theta function is a modular form of weight n/2 (on an appropriately defined subgroup) of the modular group. In the Fourier expansion,

the numbers RF(k) are called the representation numbers of the form.

Ramanujan theta function[edit]

Riemann theta function[edit]

Let

be set of symmetric square matrices whose imaginary part is positive definite. Hn is called the Siegel upper half-space and is the multi-dimensional analog of the upper half-plane. The n-dimensional analogue of the modular group is the symplectic group Sp(2n,Z); for n = 1, Sp(2,Z) = SL(2,Z). The n-dimensional analog of the congruence subgroups is played by .

Then, given , the Riemann theta function is defined as

Here, is an n-dimensional complex vector, and the superscript T denotes the transpose. The Jacobi theta function is then a special case, with n = 1 and where is the upper half-plane.

The Riemann theta converges absolutely and uniformly on compact subsets of

The functional equation is

which holds for all vectors , and for all and .

Poincaré series[edit]

The Poincaré series generalizes the theta series to automorphic forms with respect to arbitrary Fuchsian groups.

Notes[edit]

  1. ^ Tyurin, Andrey N. (30 October 2002). "Quantization, Classical and Quantum Field Theory and Theta - Functions". arXiv:math/0210466. Retrieved 31 May 2016. 
  2. ^ Jinhee, Yi (2004), "Theta-function identities and the explicit formulas for theta-function and their applications", Journal of Mathematical Analysis and Applications 292: 381–400, doi:10.1016/j.jmaa.2003.12.009. 
  3. ^ Mező, István (2013), "Duplication formulae involving Jacobi theta functions and Gosper's q-trigonometric functions", Proceedings of the American Mathematical Society 141 (7): 2401–2410, doi:10.1090/s0002-9939-2013-11576-5 
  4. ^ Mező, István (2012). "A q-Raabe formula and an integral of the fourth Jacobi theta function". Journal of Number Theory 133 (2): 692–704. doi:10.1016/j.jnt.2012.08.025. 

References[edit]

Further reading[edit]

External links[edit]

This article incorporates material from Integral representations of Jacobi theta functions on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.