# Theta-function

-function, of one complex variable

A quasi-doubly-periodic entire function of a complex variable , that is, a function having, apart from a period , also a quasi-period , , the addition of which to the argument multiplies the value of the function by a certain factor. In other words, one has the identities (in ):

As a periodic entire function, a theta-function can always be represented by a series

 (1)

where the coefficients must be chosen so as to ensure convergence. The series (1) is called a theta-series (because of the original notation). Other representations of theta-functions, for example as infinite products, are also possible.

In applications one usually restricts oneself to multipliers of the form

where is a natural number, called the order or the weight of the theta-function, and is a number. Convergence is ensured, for example, by using coefficients of the form

In many problems it is convenient to take the theta-functions that satisfy the conditions

 (2)

All theta-functions of the form (2) of the same order form a vector space of dimension . A basis for this vector space can be written in the form

Individual examples of theta-functions are already encountered in the work of J. Bernoulli (1713), L. Euler, and in the theory of heat conduction of J. Fourier. C.G.J. Jacobi subjected theta-functions to a systematic investigation, and picked out four special theta-functions, which formed the basis of his theory of elliptic functions (cf. Jacobi elliptic functions).

Theta-functions of several complex variables arise as a natural generalization of theta-functions of one complex variable. They are constructed in the following way. Let be a row-matrix of complex variables, , let be the -th row of the identity matrix of order , let be an integer row-matrix, and let be a symmetric complex matrix of order such that the matrix gives rise to a positive-definite quadratic form . (Here is the transpose of the matrix .) The multiple theta-series

 (3)

converges absolutely and uniformly on compacta in , and hence defines an entire transcendental function of complex variables , called a theta-function of order . The individual elements of the matrix are called moduli, or parameters, of the theta-function . The number of moduli is equal to . A theta-function of the first order satisfies the following basic identities (in ):

 (4)

where , and for and for . The -matrix is the moduli system or system of periods and quasi-periods of . If , are arbitrary integer row-matrices, then the periodicity property of theta-functions can be written in its most general form as

Let , be arbitrary complex row-matrices, and let be the -matrix

Then the formula

defines a theta-function of order with characteristic (in general form) . In this terminology the theta-function (3) has characteristic 0. The matrix is also called the periodicity characteristic of the matrix . One always has . Property (4) generalizes to theta-functions of characteristic :

 (5)

The characteristic is said to be normal if for .

The most commonly used are fractional characteristics, where all the and are non-negative proper fractions with common denominator . The simplest and most important case is of semi-integer or half characteristics, where . A semi-integer characteristic

can be thought of as being made up of the numbers 0 and 1 (usually a "theta-characteristictheta-characteristic" is used to mean just such a characteristic). For a theta-function with characteristic equations (5) take the form

A theta-characteristic is called even or odd, depending on whether the theta-function is even or odd. In other words, the theta-characteristic is even or odd, depending on whether the number is even or odd, since

There are distinct theta-characteristics, of which are even and are odd. The theta-function takes the value zero at those points whose theta-characteristic

yields an odd theta-characteristic when added to . Jacobi used theta-functions with semi-integer characteristics in his theory of elliptic functions, except that his had period rather than 1.

Let be a natural number. An entire transcendental function is called a theta-function of order with characteristic if it satisfies the identities

For example, the product of theta-functions of order 1 is a theta-function of order .

Using theta-functions of order with semi-integer characteristics one can construct meromorphic Abelian functions with periods. The periods of an arbitrary Abelian function in complex variables satisfy the Riemann–Frobenius relations, which yield convergence for the series defining the theta-functions with the corresponding system of moduli. According to a theorem formulated by K. Weierstrass and proved by H. Poincaré, an Abelian function can be represented as a quotient of entire theta-functions with corresponding moduli system. For the solution of the Jacobi inversion problem on Abelian integrals, one constructs a special Riemann theta-function, whose argument is a system of points on a Riemann surface.

#### References

 [1] N.G. Chebotarev, "The theory of algebraic functions" , Moscow-Leningrad (1948) pp. Chapt. 9 (In Russian) [2] A. Hurwitz, R. Courant, "Vorlesungen über allgemeine Funktionentheorie und elliptische Funktionen" , 1 , Springer (1964) pp. Chapt.8 MR0173749 Zbl 0135.12101 [3] A. Krazer, "Lehrbuch der Theta-Funktionen" , Chelsea, reprint (1970) [4] F. Conforto, "Abelsche Funktionen und algebraische Geometrie" , Springer (1956) MR0079316 Zbl 0074.36601