Theta function

One example of a theta function is: \theta(z,q) \equiv \sum_{n=-\infty}^\infty q^{n^2} \exp{(2\pi inz)} where z and q are complex numbers and |q| \theta(0,q) = \sum_{n=-\infty}^\infty q^{n^2} = 1 + 2q + 2q^4 + 2q^9 + \ldots + 2q^{n^2} + \ldots This is a generating function where the coefficient on q^k represents how many ways there are to write k as a perfect square—when k=0, there is just one way. When k is any other perfect square, there are two ways: n^2 = (-n)^2. When k is not a perfect square, there are zero ways. If you square this generating function, you obtain \theta(0,q)^2 = \Bigl(\sum_m q^{m^2}\Bigr)\Bigl(\sum_n q^{n^2}\Bigr) = \sum_{m,n} q^{m^2+n^2}. If you collect terms by exponent, you find that \theta(0,q)^2 is a generating function where the coefficient on q^k counts how many ways there are to write k as the sum of any two squares. This count includes negative integers and order, such that (3,4), (4,3), and (-3,4) each count as separate ways of making 32 + 42 = 25. Application to elliptic functions 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 ), 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 quasiperiodicity comes from the cohomology class of a line bundle on a complex torus, a condition of descent. One interpretation of theta functions when dealing with the heat equation is that "a theta function is a special function that describes the evolution of temperature on a segment domain subject to certain boundary conditions". Throughout this article, (e^{\pi i\tau})^{\alpha} should be interpreted as e^{\alpha \pi i\tau} (in order to resolve issues of choice of branch). ==Jacobi theta function==

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 and , where can be any complex number and is the half-period ratio, confined to the upper half-plane, which means it has a positive imaginary part. It is given by the formula :\begin{align} \vartheta(z; \tau) &= \sum_{n=-\infty}^\infty \exp \left(\pi i n^2 \tau + 2 \pi i n z\right) \\ &= 1 + 2 \sum_{n=1}^\infty q^{n^2} \cos(2\pi n z) \\ &= \sum_{n=-\infty}^\infty q^{n^2}\eta^n \end{align} where is the nome and . It is a Jacobi form. The restriction ensures that it is an absolutely convergent series. At fixed , this is a Fourier series for a 1-periodic entire function of . Accordingly, the theta function is 1-periodic in : :\vartheta(z+1; \tau) = \vartheta(z; \tau). By completing the square, it is also -quasiperiodic in , with :\vartheta(z+\tau;\tau) = \exp\bigl(-\pi i (\tau + 2 z)\bigr) \vartheta(z;\tau). Thus, in general, :\vartheta(z+a+b\tau;\tau) = \exp\left(-\pi i b^2 \tau -2 \pi i b z\right) \vartheta(z;\tau) for any integers and . For any fixed \tau , the function is an entire function on the complex plane, so by Liouville's theorem, it cannot be doubly periodic in 1, \tau unless it is constant, and so the best we can do is to make it periodic in 1 and quasi-periodic in \tau . Indeed, since \left|\frac{\vartheta(z+a+b\tau;\tau)}{\vartheta(z;\tau)}\right| = \exp\left(\pi (b^2 \Im(\tau) + 2b \Im(z)) \right) and \Im(\tau)> 0 , the function \vartheta(z, \tau) is unbounded, as required by Liouville's theorem. It is in fact the most general entire function with 2 quasi-periods, in the following sense: {{Math theorem \begin{cases} f(z+1) = f(z)\\ f(z + \tau) = e^{az + 2\pi i b} f(z) \end{cases} for some constant a, b\in \mathbb C. If a = 0, then b = \tau and f(z) = e^{2\pi i z}. If a = -2\pi i, then f(z) = C \vartheta(z + \frac 1 2 \tau + b, \tau) for some nonzero C\in \mathbb C. }} ==Auxiliary functions==

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: :\vartheta_{00}(z;\tau) = \vartheta(z;\tau) The auxiliary (or half-period) functions are defined by :\begin{align} \vartheta_{01}(z;\tau)& = \vartheta \left(z+\tfrac12;\tau\right)\\[3pt] \vartheta_{10}(z;\tau)& = \exp\left(\tfrac14\pi i \tau + \pi i z\right)\vartheta\left(z + \tfrac12\tau;\tau\right)\\[3pt] \vartheta_{11}(z;\tau)& = \exp\left(\tfrac14\pi i \tau + \pi i\left(z+\tfrac12\right)\right)\vartheta\left(z+\tfrac12\tau + \tfrac12;\tau\right). \end{align} This notation follows Riemann and Mumford; Jacobi's original formulation was in terms of the nome rather than . In Jacobi's notation the -functions are written: :\begin{align} \theta_1(z;q) &=\theta_1(\pi z,q)= -\vartheta_{11}(z;\tau)\\ \theta_2(z;q) &=\theta_2(\pi z,q)= \vartheta_{10}(z;\tau)\\ \theta_3(z;q) &=\theta_3(\pi z,q)= \vartheta_{00}(z;\tau)\\ \theta_4(z;q) &=\theta_4(\pi z,q)= \vartheta_{01}(z;\tau) \end{align} 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 in the above theta functions, we obtain four functions of only, defined on the upper half-plane. These functions are called Theta Nullwert functions, based on the German term for zero value because of the annullation of the left entry in the theta function expression. Alternatively, we obtain four functions of only, defined on the unit disk |q|. They are sometimes called theta constants: :\begin{align} \vartheta_{11}(0;\tau)&=-\theta_1(q)=-\sum_{n=-\infty}^\infty (-1)^{n-1/2}q^{(n+1/2)^2} \\ \vartheta_{10}(0;\tau)&=\theta_2(q)=\sum_{n=-\infty}^\infty q^{(n+1/2)^2}\\ \vartheta_{00}(0;\tau)&=\theta_3(q)=\sum_{n=-\infty}^\infty q^{n^2}\\ \vartheta_{01}(0;\tau)&=\theta_4(q)=\sum_{n=-\infty}^\infty (-1)^n q^{n^2} \end{align} with the nome . Observe that \theta_1(q)=0 . These can be used to define a variety of modular forms, and to parametrize certain curves; in particular, the Jacobi identity is :\theta_2(q)^4 + \theta_4(q)^4 = \theta_3(q)^4 or equivalently, :\vartheta_{01}(0;\tau)^4 + \vartheta_{10}(0;\tau)^4 = \vartheta_{00}(0;\tau)^4 which is the Fermat curve of degree four. == Jacobi identities ==

Jacobi's identities describe how theta functions transform under the modular group, which is generated by and . Equations for the first transform are easily found since adding one to in the exponent has the same effect as adding to (). For the second, let :\alpha = (-i \tau)^\frac12 \exp\left(\frac{\pi}{\tau} i z^2 \right). Then :\begin{align} \vartheta_{00}\!\left(\frac{z}{\tau}; \frac{-1}{\tau}\right)& = \alpha\,\vartheta_{00}(z; \tau)\quad& \vartheta_{01}\!\left(\frac{z}{\tau}; \frac{-1}{\tau}\right)& = \alpha\,\vartheta_{10}(z; \tau)\\[3pt] \vartheta_{10}\!\left(\frac{z}{\tau}; \frac{-1}{\tau}\right)& = \alpha\,\vartheta_{01}(z; \tau)\quad& \vartheta_{11}\!\left(\frac{z}{\tau}; \frac{-1}{\tau}\right)& = -i\alpha\,\vartheta_{11}(z; \tau). \end{align} ==Theta functions in terms of the nome==

Instead of expressing the Theta functions in terms of and , we may express them in terms of arguments and the nome , where and . In this form, the functions become :\begin{align} \vartheta_{00}(w, q)& = \sum_{n=-\infty}^\infty \left(w^2\right)^n q^{n^2}\quad& \vartheta_{01}(w, q)& = \sum_{n=-\infty}^\infty (-1)^n \left(w^2\right)^n q^{n^2}\\[3pt] \vartheta_{10}(w, q)& = \sum_{n=-\infty}^\infty \left(w^2\right)^{n+\frac12} q^{\left(n + \frac12\right)^2}\quad& \vartheta_{11}(w, q)& = i \sum_{n=-\infty}^\infty (-1)^n \left(w^2\right)^{n+\frac12} q^{\left(n + \frac12\right)^2}. \end{align} We see that the theta functions can also be defined in terms of and , 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 -adic numbers. ==Product representations==

The Jacobi triple product (a special case of the Macdonald identities) tells us that for complex numbers and with and we have :\prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 + w^2 q^{2m-1}\right) \left( 1 + w^{-2}q^{2m-1}\right) = \sum_{n=-\infty}^\infty w^{2n}q^{n^2}. 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 (noting some authors instead set ) and take then :\vartheta(z; \tau) = \sum_{n=-\infty}^\infty \exp(\pi i \tau n^2) \exp(2\pi i z n) = \sum_{n=-\infty}^\infty w^{2n}q^{n^2}. We therefore obtain a product formula for the theta function in the form :\vartheta(z; \tau) = \prod_{m=1}^\infty \big( 1 - \exp(2m \pi i \tau)\big) \Big( 1 + \exp\big((2m-1) \pi i \tau + 2 \pi i z\big)\Big) \Big( 1 + \exp\big((2m-1) \pi i \tau - 2 \pi i z\big)\Big). In terms of and : :\begin{align} \vartheta(z; \tau) &= \prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 + q^{2m-1}w^2\right) \left( 1 + \frac{q^{2m-1}}{w^2}\right) \\ &= \left(q^2;q^2\right)_\infty\,\left(-w^2q;q^2\right)_\infty\,\left(-\frac{q}{w^2};q^2\right)_\infty \\ &= \left(q^2;q^2\right)_\infty\,\theta\left(-w^2q;q^2\right) \end{align} where is the -Pochhammer symbol and is the -theta function. Expanding terms out, the Jacobi triple product can also be written :\prod_{m=1}^\infty \left( 1 - q^{2m}\right) \Big( 1 + \left(w^2+w^{-2}\right)q^{2m-1}+q^{4m-2}\Big), which we may also write as :\vartheta(z\mid q) = \prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 + 2 \cos(2 \pi z)q^{2m-1}+q^{4m-2}\right). This form is valid in general but clearly is of particular interest when is real. Similar product formulas for the auxiliary theta functions are :\begin{align} \vartheta_{01}(z\mid q) &= \prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 - 2 \cos(2 \pi z)q^{2m-1}+q^{4m-2}\right),\\[3pt] \vartheta_{10}(z\mid q) &= 2 q^\frac14\cos(\pi z)\prod_{m=1}^\infty \left( 1 - q^{2m}\right) \left( 1 + 2 \cos(2 \pi z)q^{2m}+q^{4m}\right),\\[3pt] \vartheta_{11}(z\mid q) &= -2 q^\frac14\sin(\pi z)\prod_{m=1}^\infty \left( 1 - q^{2m}\right)\left( 1 - 2 \cos(2 \pi z)q^{2m}+q^{4m}\right). \end{align} In particular, \lim_{q\to 0}\frac{\vartheta_{10}(z\mid q)}{2 q^{\frac14}} = \cos(\pi z),\quad \lim_{q\to 0}\frac{-\vartheta_{11}(z\mid q)}{2 q^{\frac14}} = \sin(\pi z)so we may interpret them as one-parameter deformations of the periodic functions \sin, \cos, again validating the interpretation of the theta function as the most general 2 quasi-period function. ==Integral representations==

The Jacobi theta functions have the following integral representations: :\begin{align} \vartheta_{00} (z; \tau) &= -i\int_{i-\infty}^{i+\infty} e^{i \pi \tau u^2} \frac{\cos(2\pi uz + \pi u)}{\sin(\pi u)} \mathrm{d}u; \\[6pt] \vartheta_{01} (z; \tau) &= -i\int_{i-\infty}^{i+\infty} e^{i \pi \tau u^2} \frac{\cos (2\pi uz)}{\sin (\pi u)} \mathrm{d}u; \\[6pt] \vartheta_{10} (z; \tau) &= -ie^{i \pi z + \frac14 i \pi\tau} \int_{i-\infty}^{i+\infty} e^{i\pi\tau u^2} \frac{\cos(2\pi uz + \pi u + \pi \tau u)}{\sin (\pi u)} \mathrm{d}u; \\[6pt] \vartheta_{11} (z; \tau) &= e^{i \pi z + \frac14 i \pi \tau} \int_{i-\infty}^{i+\infty} e^{i\pi\tau u^2} \frac{\cos(2\pi uz + \pi \tau u)}{\sin(\pi u)} \mathrm{d}u. \end{align} The Theta Nullwert function \theta_{3}(q) as this integral identity: :\theta_{3}(q) = 1 + \frac{4q\sqrt{\ln(1/q)}}{\sqrt{\pi}} \int_{0}^{\infty} \frac{\exp[-\ln(1/q)\,x^2]\{1 - q^2\cos[2\ln(1/q)\,x]\}}{1 - 2q^2\cos[2\ln(1/q)\,x] + q^4} \,\mathrm{d}x This formula was discussed in the essay Square series generating function transformations by the mathematician Maxie Schmidt from Georgia in Atlanta. Based on this formula following three eminent examples are given: :\biggl[\frac{2}{\pi}K\bigl(\frac{1}{2}\sqrt{2}\bigr)\biggr]^{1/2} = \theta_{3}\bigl[\exp(-\pi)\bigr] = 1 + 4\exp(-\pi) \int_{0}^{\infty} \frac{\exp(-\pi x^2)[1 - \exp(-2\pi)\cos(2\pi x)]}{1 - 2\exp(-2\pi)\cos(2\pi x) + \exp(-4\pi)} \,\mathrm{d}x :\biggl[\frac{2}{\pi}K(\sqrt{2} - 1)\biggr]^{1/2} = \theta_{3}\bigl[\exp(-\sqrt{2}\,\pi)\bigr] = 1 + 4\,\sqrt[4]{2}\exp(-\sqrt{2}\,\pi) \int_{0}^{\infty} \frac{\exp(-\sqrt{2}\,\pi x^2)[1 - \exp(-2\sqrt{2}\,\pi)\cos(2\sqrt{2}\,\pi x)]}{1 - 2\exp(-2\sqrt{2}\,\pi)\cos(2\sqrt{2}\,\pi x) + \exp(-4\sqrt{2}\,\pi)} \,\mathrm{d}x :\biggl\{\frac{2}{\pi}K\bigl[\sin\bigl(\frac{\pi}{12}\bigr)\bigr]\biggr\}^{1/2} = \theta_{3}\bigl[\exp(-\sqrt{3}\,\pi)\bigr] = 1 + 4\,\sqrt[4]{3}\exp(-\sqrt{3}\,\pi) \int_{0}^{\infty} \frac{\exp(-\sqrt{3}\,\pi x^2)[1 - \exp(-2\sqrt{3}\,\pi)\cos(2\sqrt{3}\,\pi x)]}{1 - 2\exp(-2\sqrt{3}\,\pi)\cos(2\sqrt{3}\,\pi x) + \exp(-4\sqrt{3}\,\pi)} \,\mathrm{d}x Furthermore, the theta examples \theta_{3}(\tfrac{1}{2}) and \theta_{3}(\tfrac{1}{3}) shall be displayed: :\theta_{3}\left(\frac{1}{2}\right) = 1+2\sum_{n = 1}^{\infty} \frac{1}{2^{n^2}} = 1 + 2\pi^{-1/2}\sqrt{\ln(2)} \int_{0}^{\infty} \frac{\exp[-\ln(2)\,x^2]\{16 - 4\cos[2\ln(2)\,x]\}}{17 - 8\cos[2\ln(2)\,x]} \,\mathrm{d}x :\theta_{3}\left(\frac{1}{2}\right) = 2.128936827211877158669\ldots :\theta_{3}\left(\frac{1}{3}\right) = 1+2\sum_{n = 1}^{\infty} \frac{1}{3^{n^2}} = 1 + \frac{4}{3}\pi^{-1/2}\sqrt{\ln(3)} \int_{0}^{\infty} \frac{\exp[-\ln(3)\,x^2]\{81 - 9\cos[2\ln(3)\,x]\}}{82 - 18\cos[2\ln(3)\,x]} \,\mathrm{d}x :\theta_{3}\left(\frac{1}{3}\right) = 1.691459681681715341348\ldots ==Explicit values==

=== Lemniscatic values === Proper credit for most of these results goes to Ramanujan. See Ramanujan's lost notebook and a relevant reference at Euler function. The Ramanujan results quoted at Euler function plus a few elementary operations give the results below, so they are either in Ramanujan's lost notebook or follow immediately from it. See also Yi (2004). Define, :\quad \varphi(q) =\vartheta_{00}(0;\tau) =\theta_3(0;q)=\sum_{n=-\infty}^\infty q^{n^2} with the nome q =e^{\pi i \tau}, \tau = n\sqrt{-1}, and Dedekind eta function \eta(\tau). Then for n = 1,2,3,\dots :\begin{align} \varphi\left(e^{-\pi} \right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} = \sqrt2\,\eta\left(\sqrt{-1}\right)\\ \varphi\left(e^{-2\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{2+\sqrt2}}{2}\\ \varphi\left(e^{-3\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{1+\sqrt3}}{\sqrt[8]{108}}\\ \varphi\left(e^{-4\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{2+\sqrt[4]{8}}{4}\\ \varphi\left(e^{-5\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \sqrt{\frac{2+\sqrt5}{5}}\\ \varphi\left(e^{-6\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{\sqrt[4]{1}+\sqrt[4]{3}+\sqrt[4]{4}+\sqrt[4]{9}}}{\sqrt[8]{12^3}}\\ \varphi\left(e^{-7\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{\sqrt{13+\sqrt{7}}+\sqrt{7+3\sqrt{7}}}}{\sqrt[8]{14^3}\cdot\sqrt[16]{7}}\\ \varphi\left(e^{-8\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{2+\sqrt{2}}+\sqrt[8]{128}}{4}\\ \varphi\left(e^{-9\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{1+\sqrt[3]{2+2\sqrt{3}}}{3}\\ \varphi\left(e^{-10\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{\sqrt[4]{64}+\sqrt[4]{80}+\sqrt[4]{81}+\sqrt[4]{100}}}{\sqrt[4]{200}}\\ \varphi\left(e^{-11\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{11+\sqrt{11}+(5+3\sqrt{3}+\sqrt{11}+\sqrt{33})\sqrt[3]{-44+33\sqrt{3}}+(-5+3\sqrt{3}-\sqrt{11}+\sqrt{33})\sqrt[3]{44+33\sqrt{3}}}}{\sqrt[8]{52180524}}\\ \varphi\left(e^{-12\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{\sqrt[4]{1}+\sqrt[4]{2}+\sqrt[4]{3}+\sqrt[4]{4}+\sqrt[4]{9}+\sqrt[4]{18}+\sqrt[4]{24}}}{2\sqrt[8]{108}}\\ \varphi\left(e^{-13\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{13+8\sqrt{13}+(11-6\sqrt{3}+\sqrt{13})\sqrt[3]{143+78\sqrt{3}}+(11+6\sqrt{3}+\sqrt{13})\sqrt[3]{143-78\sqrt{3}}}}{\sqrt[4]{19773}}\\ \varphi\left(e^{-14\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{\sqrt{13+\sqrt{7}}+\sqrt{7+3\sqrt{7}}+\sqrt{10+2\sqrt{7}}+\sqrt[8]{28}\sqrt{4+\sqrt{7}}}}{\sqrt[16]{28^7}}\\ \varphi\left(e^{-15\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{7+3\sqrt{3}+\sqrt{5}+\sqrt{15}+\sqrt[4]{60}+\sqrt[4]{1500}}}{\sqrt[8]{12^3}\cdot\sqrt{5}}\\ 2\varphi\left(e^{-16\pi}\right) &= \varphi\left(e^{-4\pi}\right) + \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt[4]{1+\sqrt{2}}}{\sqrt[16]{128}}\\ \varphi\left(e^{-17\pi}\right) &= \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \frac{\sqrt{2}(1+\sqrt[4]{17})+\sqrt[8]{17}\sqrt{5+\sqrt{17}}}{\sqrt{17+17\sqrt{17}}}\\ 2\varphi\left(e^{-20\pi}\right) &= \varphi\left(e^{-5\pi}\right) + \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \sqrt{\frac{3+2\sqrt[4]{5}}{5\sqrt2}}\\ 6\varphi\left(e^{-36\pi}\right) &= 3\varphi\left(e^{-9\pi}\right) + 2\varphi\left(e^{-4\pi}\right) - \varphi\left(e^{-\pi}\right) + \frac{\sqrt[4]{\pi}}{\Gamma\left(\frac34\right)} \sqrt[3]{\sqrt[4]{2}+\sqrt[4]{18}+\sqrt[4]{216}} \end{align} If the reciprocal of the Gelfond constant is raised to the power of the reciprocal of an odd number, then the corresponding \vartheta_{00} values or \phi values can be represented in a simplified way by using the hyperbolic lemniscatic sine: : \varphi\bigl[\exp(-\tfrac{1}{5}\pi)\bigr] = \sqrt[4]{\pi}\,{\Gamma\left(\tfrac{3}{4}\right)}^{-1} \operatorname{slh}\bigl(\tfrac{1}{5}\sqrt{2}\,\varpi\bigr)\operatorname{slh}\bigl(\tfrac{2}{5}\sqrt{2}\,\varpi\bigr) : \varphi\bigl[\exp(-\tfrac{1}{7}\pi)\bigr] = \sqrt[4]{\pi}\,{\Gamma\left(\tfrac{3}{4}\right)}^{-1} \operatorname{slh}\bigl(\tfrac{1}{7}\sqrt{2}\,\varpi\bigr)\operatorname{slh}\bigl(\tfrac{2}{7}\sqrt{2}\,\varpi\bigr) \operatorname{slh}\bigl(\tfrac{3}{7}\sqrt{2}\,\varpi\bigr) : \varphi\bigl[\exp(-\tfrac{1}{9}\pi)\bigr] = \sqrt[4]{\pi}\,{\Gamma\left(\tfrac{3}{4}\right)}^{-1} \operatorname{slh}\bigl(\tfrac{1}{9}\sqrt{2}\,\varpi\bigr)\operatorname{slh}\bigl(\tfrac{2}{9}\sqrt{2}\,\varpi\bigr) \operatorname{slh}\bigl(\tfrac{3}{9}\sqrt{2}\,\varpi\bigr) \operatorname{slh} \bigl(\tfrac{4}{9}\sqrt{2}\,\varpi\bigr) : \varphi\bigl[\exp(-\tfrac{1}{11}\pi)\bigr] = \sqrt[4]{\pi}\,{\Gamma\left(\tfrac{3}{4}\right)}^{-1} \operatorname{slh}\bigl(\tfrac{1}{11}\sqrt{2}\,\varpi\bigr)\operatorname{slh}\bigl(\tfrac{2}{11}\sqrt{2}\,\varpi\bigr) \operatorname{slh}\bigl(\tfrac{3}{11}\sqrt{2}\,\varpi\bigr) \operatorname{slh} \bigl(\tfrac{4}{11}\sqrt{2}\,\varpi\bigr) \operatorname{slh}\bigl(\tfrac{5}{11}\sqrt{2}\,\varpi\bigr) With the letter \varpi the Lemniscate constant is represented. Note that the following modular identities hold: :\begin{align} 2\varphi\left(q^4\right) &= \varphi(q)+\sqrt{2\varphi^2\left(q^2\right)-\varphi^2(q)}\\ 3\varphi\left(q^9\right) &= \varphi(q)+\sqrt[3]{9\frac{\varphi^4\left(q^3\right)}{\varphi(q)}-\varphi^3(q)}\\ \sqrt{5}\varphi\left(q^{25}\right) &= \varphi\left(q^5\right)\cot\left(\frac{1}{2}\arctan\left(\frac{2}{\sqrt{5}}\frac{\varphi(q)\varphi\left(q^5\right)}{\varphi^2(q)-\varphi^2\left(q^5\right)}\frac{1+s(q)-s^2(q)}{s(q)}\right)\right) \end{align} where s(q)=s\left(e^{\pi i\tau}\right)=-R\left(-e^{-\pi i/(5\tau)}\right) is the Rogers–Ramanujan continued fraction: :\begin{align} s(q) &= \sqrt[5]{\tan\left(\frac{1}{2}\arctan\left(\frac{5}{2}\frac{\varphi^2\left(q^5\right)}{\varphi^2(q)}-\frac{1}{2}\right)\right)\cot^2\left(\frac{1}{2}\operatorname{arccot}\left(\frac{5}{2}\frac{\varphi^2\left(q^5\right)}{\varphi^2(q)}-\frac{1}{2}\right)\right)}\\ &= \cfrac{e^{-\pi i/(25\tau)}}{1-\cfrac{e^{-\pi i/(5\tau)}}{1+\cfrac{e^{-2\pi i/(5\tau)}}{1-\ddots}}} \end{align} === Equianharmonic values === The mathematician Bruce Berndt found out further values of the theta function: :\begin{array}{lll} \varphi\left(\exp( -\sqrt{3}\,\pi)\right) &=& \pi^{-1}{\Gamma\left(\tfrac{4}{3}\right)}^{3/2}2^{-2/3}3^{13/8} \\ \varphi\left(\exp(-2\sqrt{3}\,\pi)\right) &=& \pi^{-1}{\Gamma\left(\tfrac{4}{3}\right)}^{3/2}2^{-2/3}3^{13/8}\cos(\tfrac{1}{24}\pi) \\ \varphi\left(\exp(-3\sqrt{3}\,\pi)\right) &=& \pi^{-1}{\Gamma\left(\tfrac{4}{3}\right)}^{3/2}2^{-2/3}3^{7/8}(\sqrt[3]{2}+1) \\ \varphi\left(\exp(-4\sqrt{3}\,\pi)\right) &=& \pi^{-1}{\Gamma\left(\tfrac{4}{3}\right)}^{3/2}2^{-5/3}3^{13/8}\Bigl(1+\sqrt{\cos(\tfrac{1}{12}\pi)}\Bigr) \\ \varphi\left(\exp(-5\sqrt{3}\,\pi)\right) &=& \pi^{-1}{\Gamma\left(\tfrac{4}{3}\right)}^{3/2}2^{-2/3}3^{5/8}\sin(\tfrac{1}{5}\pi)(\tfrac{2}{5}\sqrt[3]{100}+\tfrac{2}{5}\sqrt[3]{10}+\tfrac{3}{5}\sqrt{5}+1) \end{array} Further values Many values of the theta function and especially of the shown phi function can be represented in terms of the gamma function: :\begin{array}{lll} \varphi\left(\exp( -\sqrt{2}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{9}{8}\right){\Gamma\left(\tfrac{5}{4}\right)}^{-1/2}2^{7/8} \\ \varphi\left(\exp(-2\sqrt{2}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{9}{8}\right){\Gamma\left(\tfrac{5}{4}\right)}^{-1/2}2^{1/8}\Bigl(1+\sqrt{\sqrt{2}-1}\Bigr) \\ \varphi\left(\exp(-3\sqrt{2}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{9}{8}\right){\Gamma\left(\tfrac{5}{4}\right)}^{-1/2}2^{3/8}3^{-1/2}(\sqrt{3}+1)\sqrt{\tan(\tfrac{5}{24}\pi)} \\ \varphi\left(\exp(-4\sqrt{2}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{9}{8}\right){\Gamma\left(\tfrac{5}{4}\right)}^{-1/2}2^{-1/8}\Bigl(1+\sqrt[4]{2\sqrt{2}-2}\Bigr) \\ \varphi\left(\exp(-5\sqrt{2}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{9}{8}\right){\Gamma\left(\tfrac{5}{4}\right)}^{-1/2} \frac{1}{15}\,2^{3/8} \times \\ && \times \biggl[\sqrt[3]{5}\,\sqrt{10+2\sqrt{5}}\biggl(\sqrt[3]{5+\sqrt{2}+3\sqrt{3}}+\sqrt[3]{5+\sqrt{2}-3\sqrt{3}}\,\biggr)-\bigl(2-\sqrt{2}\,\bigr)\sqrt{25-10\sqrt{5}}\,\biggr] \\ \varphi\left(\exp( -\sqrt{6}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{5}{24}\right){\Gamma\left(\tfrac{5}{12}\right)}^{-1/2}2^{-13/24}3^{-1/8}\sqrt{\sin(\tfrac{5}{12}\pi)} \\ \varphi\left(\exp(-\tfrac{1}{2}\sqrt{6}\,\pi)\right) &=& \pi^{-1/2}\Gamma\left(\tfrac{5}{24}\right){\Gamma\left(\tfrac{5}{12}\right)}^{-1/2}2^{5/24}3^{-1/8}\sin(\tfrac{5}{24}\pi) \end{array} ==Nome power theorems==

Direct power theorems For the transformation of the nome in the theta functions these formulas can be used: :\theta_{2}(q^2) = \tfrac{1}{2}\sqrt{2[\theta_{3}(q)^2 - \theta_{4}(q)^2]} :\theta_{3}(q^2) = \tfrac{1}{2}\sqrt{2[\theta_{3}(q)^2 + \theta_{4}(q)^2]} :\theta_{4}(q^2) = \sqrt{\theta_{4}(q)\theta_{3}(q)} The squares of the three theta zero-value functions with the square function as the inner function are also formed in the pattern of the Pythagorean triples according to the Jacobi identity. Furthermore, those transformations are valid: :\theta_{3}(q^4) = \tfrac{1}{2}\theta_{3}(q) + \tfrac{1}{2}\theta_{4}(q) These formulas can be used to compute the theta values of the cube of the nome: :27\,\theta_{3}(q^3)^8 - 18\,\theta_{3}(q^3)^4\theta_{3}(q)^4 - \,\theta_{3}(q)^8 = 8\,\theta_{3}(q^3)^2\theta_{3}(q)^2[2\,\theta_{4}(q)^4 - \theta_{3}(q)^4] :27\,\theta_{4}(q^3)^8 - 18\,\theta_{4}(q^3)^4\theta_{4}(q)^4 - \,\theta_{4}(q)^8 = 8\,\theta_{4}(q^3)^2\theta_{4}(q)^2[2\,\theta_{3}(q)^4 - \theta_{4}(q)^4] And the following formulas can be used to compute the theta values of the fifth power of the nome: :[\theta_{3}(q)^2 - \theta_{3}(q^5)^2][5\,\theta_{3}(q^5)^2 - \theta_{3}(q)^2]^5 = 256\,\theta_{3}(q^5)^2\theta_{3}(q)^2\theta_{4}(q)^4 [\theta_{3}(q)^4 - \theta_{4}(q)^4] :[\theta_{4}(q^5)^2 - \theta_{4}(q)^2][5\,\theta_{4}(q^5)^2 - \theta_{4}(q)^2]^5 = 256\,\theta_{4}(q^5)^2\theta_{4}(q)^2\theta_{3}(q)^4 [\theta_{3}(q)^4 - \theta_{4}(q)^4] Transformation at the cube root of the nome The formulas for the theta Nullwert function values from the cube root of the elliptic nome are obtained by contrasting the two real solutions of the corresponding quartic equations: : \biggl[\frac{\theta_{3}(q^{1/3})^2}{\theta_{3}(q)^2} - \frac{3\,\theta_{3}(q^{3})^2}{\theta_{3}(q)^2}\biggr]^2 = 4 - 4\biggl[\frac{2\,\theta_{2}(q)^2 \theta_{4}(q)^2}{\theta_{3}(q)^4}\biggr]^{2/3} : \biggl[\frac{3\,\theta_{4}(q^{3})^2}{\theta_{4}(q)^2} - \frac{\theta_{4}(q^{1/3})^2}{\theta_{4}(q)^2}\biggr]^2 = 4 + 4\biggl[\frac{2\,\theta_{2}(q)^2 \theta_{3}(q)^2}{\theta_{4}(q)^4}\biggr]^{2/3} Transformation at the fifth root of the nome The Rogers-Ramanujan continued fraction can be defined in terms of the Jacobi theta function in the following way: : R(q) = \tan\biggl\{\frac{1}{2}\arctan\biggl[\frac{1}{2} - \frac{\theta _{4}(q)^2}{2\,\theta_{4}(q^5)^2}\biggr]\biggr\}^{1/5} \tan\biggl\{\frac{1}{2}\arccot\biggl[\frac{1}{2} - \frac{\theta_{4}(q)^2}{2\,\theta_{4}(q^5)^2}\biggr]\biggr\}^{2/5} : R(q^2) = \tan\biggl\{\frac{1}{2}\arctan\biggl[\frac{1}{2} - \frac{\theta_{4}(q)^2}{2\,\theta_{4}(q^5)^2}\biggr]\biggr\}^{2/5} \cot\biggl\{\frac{1}{2}\arccot\biggl[\frac{1}{2} - \frac{\theta_{4}(q)^2}{2\,\theta_{4}(q^5)^2}\biggr]\biggr\}^{1/5} : R(q^2) = \tan\biggl\{\frac{1}{2}\arctan\biggl[\frac{\theta_{3}(q)^2}{2\,\theta_{3}(q^5)^2} - \frac{1}{2}\biggr]\biggr\}^{2/5} \tan\biggl\{\frac{1}{2}\arccot\biggl[\frac{\theta_{3}(q)^2}{2\,\theta_{3}(q^5)^2} - \frac{1}{2}\biggr]\biggr\}^{1/5} The alternating Rogers-Ramanujan continued fraction function S(q) has the following two identities: : S(q) = \frac{R(q^4)}{R(q^2)R(q)} = \tan\biggl\{\frac{1}{2}\arctan\biggl [\frac{\theta_{3}(q)^2}{2\,\theta_{3}(q^5)^2} - \frac{1}{2}\biggr]\biggr\}^{1/5} \cot\biggl\{\frac{1}{2}\arccot\biggl[\frac{\theta_{3}(q)^2}{2\,\theta_{3}(q^5)^2} - \frac{1}{2}\biggr]\biggr\}^{2/5} The theta function values from the fifth root of the nome can be represented as a rational combination of the continued fractions R and S and the theta function values from the fifth power of the nome and the nome itself. The following four equations are valid for all values q between 0 and 1: : \frac{\theta_{3}(q^{1/5})}{\theta_{3}(q^5)} - 1 = \frac{1}{S(q)}\bigl[S(q)^2 + R(q^2)\bigr]\bigl[1 + R(q^2)S(q)\bigr] : 1 - \frac{\theta_{4}(q^{1/5})}{\theta_{4}(q^5)} = \frac{1}{R(q)}\bigl[R(q^2) + R(q)^2\bigr]\bigl[1 - R(q^2)R(q)\bigr] : \theta_{3}(q^{1/5})^2 - \theta_{3}(q)^2 = \bigl[\theta_{3}(q)^2 - \theta_{3}(q^5)^2\bigr]\biggl[1+\frac{1}{R(q^2)S(q)}+R(q^2)S(q)+\frac{1}{R(q^2)^2}+R(q^2)^2+\frac{1}{S(q)}-S(q)\biggr] : \theta_{4}(q)^2 - \theta_{4}(q^{1/5})^2 = \bigl[\theta_{4}(q^5)^2 - \theta_{4}(q)^2\bigr]\biggl[1-\frac{1}{R(q^2)R(q)}-R(q^2)R(q)+\frac{1}{R(q^2)^2}+R(q^2)^2-\frac{1}{R(q)}+R(q)\biggr] Modulus dependent theorems In combination with the elliptic modulus, the following formulas can be displayed: These are the formulas for the square of the elliptic nome: :\theta_{4}[q(k)] = \theta_{4}[q(k)^2]\sqrt[8]{1 - k^2} :\theta_{4}[q(k)^2] = \theta_{3}[q(k)]\sqrt[8]{1 - k^2} :\theta_{3}[q(k)^2] = \theta_{3}[q(k)]\cos[\tfrac{1}{2}\arcsin(k)] And this is an efficient formula for the cube of the nome: : \theta_{4}\biggl\langle q\bigl\{\tan\bigl[\tfrac{1}{2}\arctan(t^3)\bigr]\bigr\}^3 \biggr\rangle = \theta_{4}\biggl\langle q\bigl\{\tan\bigl[\tfrac{1}{2}\arctan(t^3)\bigr]\bigr\} \biggr\rangle \,3^{-1/2} \bigl(\sqrt{2\sqrt{t^4 - t^2 + 1} - t^2 + 2} + \sqrt{t^2 + 1}\,\bigr)^{1/2} For all real values t \in \R the now mentioned formula is valid. And for this formula two examples shall be given: First calculation example with the value t = 1 inserted: : Second calculation example with the value t = \Phi^{-2} inserted: : The constant \Phi represents the golden ratio number \Phi = \tfrac{1}{2}(\sqrt{5} + 1) exactly. ==Some series identities==

Sums with theta function in the result The infinite sum of the reciprocals of Fibonacci numbers with odd indices has the identity: :\sum_{n=1}^\infty \frac{1}{F_{2n-1}} = \frac{\sqrt{5}}{2}\,\sum_{n=1}^\infty \frac{2(\Phi^{-2})^{n - 1/2}}{1 + (\Phi^{-2})^{2n - 1}} = \frac{\sqrt{5}}{4} \sum_{a=-\infty}^\infty \frac{2(\Phi^{-2})^{a - 1/2}}{1 + (\Phi^{-2})^{2a - 1}} = := \frac{\sqrt{5}}{4}\,\theta_{2}(\Phi^{-2})^2 = \frac{\sqrt{5}}{8}\bigl[\theta_{3}(\Phi^{-1})^2 - \theta_{4}(\Phi^{-1})^2\bigr] By not using the theta function expression, following identity between two sums can be formulated: :\sum_{n=1}^\infty \frac{1}{F_{2n-1}} = \frac{\sqrt{5}}{4}\,\biggl[ \sum_{n=1}^\infty 2 \,\Phi^{- (2n - 1)^2 /2} \biggr]^2 :\sum_{n=1}^\infty \frac{1}{F_{2n-1}} = 1.82451515740692456814215840626732817332\ldots Also in this case \Phi = \tfrac{1}{2}(\sqrt{5} + 1) is Golden ratio number again. Infinite sum of the reciprocals of the Fibonacci number squares: :\sum_{n=1}^\infty \frac{1}{F_{n}^2} = \frac{5}{24}\bigl[2\,\theta_{2}(\Phi^{-2})^4 - \theta_{3}(\Phi^{-2})^4 + 1\bigr] = \frac{5}{24}\bigl[\theta_{3}(\Phi^{-2})^4 - 2\,\theta_{4}(\Phi^{-2})^4 + 1\bigr] Infinite sum of the reciprocals of the Pell numbers with odd indices: :\sum_{n=1}^\infty \frac{1}{P_{2n-1}} = \frac{1}{\sqrt{2}}\,\theta_{2}\bigl[(\sqrt{2}-1)^2\bigr]^2 = \frac{1}{2\sqrt{2}}\bigl[\theta_{3}(\sqrt{2}-1)^2 - \theta_{4}(\sqrt{2}-1)^2\bigr] Sums with theta function in the summand The next two series identities were proved by István Mező: :\begin{align} \theta_4^2(q)&=iq^{\frac14}\sum_{k=-\infty}^\infty q^{2k^2-k}\theta_1\left(\frac{2k-1}{2i}\ln q,q\right),\\[6pt] \theta_4^2(q)&=\sum_{k=-\infty}^\infty q^{2k^2}\theta_4\left(\frac{k\ln q}{i},q\right). \end{align} These relations hold for all . Specializing the values of , we have the next parameter free sums :\sqrt{\frac{\pi\sqrt{e^\pi}}{2}}\cdot\frac{1}{\Gamma^2\left(\frac34\right)} =i\sum_{k=-\infty}^\infty e^{\pi\left(k-2k^2\right)} \theta_1 \left(\frac{i\pi}{2}(2k-1),e^{-\pi}\right) :\sqrt{\frac{\pi}{2}}\cdot\frac{1}{\Gamma^2\left(\frac34\right)} =\sum_{k=-\infty}^\infty\frac{\theta_4\left(ik\pi,e^{-\pi}\right)}{e^{2\pi k^2}} ==Zeros of the Jacobi theta functions==

All zeros of the Jacobi theta functions are simple zeros and are given by the following: :\begin{align} \vartheta(z;\tau) = \vartheta_{00}(z;\tau) &= 0 \quad &\Longleftrightarrow&& \quad z &= m + n \tau + \frac{1}{2} + \frac{\tau}{2} \\[3pt] \vartheta_{11}(z;\tau) &= 0 \quad &\Longleftrightarrow&& \quad z &= m + n \tau \\[3pt] \vartheta_{10}(z;\tau) &= 0 \quad &\Longleftrightarrow&& \quad z &= m + n \tau + \frac{1}{2} \\[3pt] \vartheta_{01}(z;\tau) &= 0 \quad &\Longleftrightarrow&& \quad z &= m + n \tau + \frac{\tau}{2} \end{align} where , are arbitrary integers. ==Relation to the Riemann zeta function==

The relation :\vartheta\left(0;-\frac{1}{\tau}\right)=\left(-i\tau\right)^\frac12 \vartheta(0;\tau) was used by Riemann to prove the functional equation for the Riemann zeta function, by means of the Mellin transform :\Gamma\left(\frac{s}{2}\right) \pi^{-\frac{s}{2}} \zeta(s) = \frac{1}{2}\int_0^\infty\bigl(\vartheta(0;it)-1\bigr)t^\frac{s}{2}\frac{\mathrm{d}t}{t} which can be shown to be invariant under substitution of by . The corresponding integral for is given in the article on the Hurwitz zeta function. ==Relation to the Weierstrass elliptic function==

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 :\wp(z;\tau) = -\big(\log \vartheta_{11}(z;\tau)\big)'' + c where the second derivative is with respect to and the constant is defined so that the Laurent expansion of at has zero constant term. ==Relation to the q-gamma function==

The fourth theta function – and thus the others too – is intimately connected to the Jackson -gamma function via the relation :\left(\Gamma_{q^2}(x)\Gamma_{q^2}(1-x)\right)^{-1}=\frac{q^{2x(1-x)}}{\left(q^{-2};q^{-2}\right)^3_\infty\left(q^2-1\right)} \theta_4\left(\frac{1}{2i}(1-2x)\log q,\frac{1}{q}\right). ==Relations to Dedekind eta function==

Let be the Dedekind eta function, and the argument of the theta function as the nome . Then, :\begin{align} \theta_2(q) = \vartheta_{10}(0;\tau) &= \frac{2\eta^2(2\tau)}{\eta(\tau)},\\[3pt] \theta_3(q) = \vartheta_{00}(0;\tau) &= \frac{\eta^5(\tau)}{\eta^2\left(\frac{1}{2}\tau\right)\eta^2(2\tau)} = \frac{\eta^2\left(\frac{1}{2}(\tau+1)\right)}{\eta(\tau+1)}, \\[3pt] \theta_4(q) = \vartheta_{01}(0;\tau) &= \frac{\eta^2\left(\frac{1}{2}\tau\right)}{\eta(\tau)}, \end{align} and, :\theta_2(q)\,\theta_3(q)\,\theta_4(q) = 2\eta^3(\tau). See also the Weber modular functions. ==Elliptic modulus==

The elliptic modulus is :k(\tau) = \frac{\vartheta_{10}(0;\tau)^2 }{\vartheta_{00}(0;\tau)^2} and the complementary elliptic modulus is :k'(\tau) = \frac{\vartheta_{01}(0;\tau)^2 }{\vartheta_{00}(0;\tau)^2} == Derivatives of theta functions ==

These are two identical definitions of the complete elliptic integral of the second kind: :E(k) = \int_{0}^{\pi/2} \sqrt{1 - k^2\sin(\varphi)^2} d\varphi :E(k) = \frac{\pi}{2}\sum_{a = 0}^{\infty} \frac{[(2a)!]^2}{(1 - 2a)16^{a}(a!)^4} k^{2a} The derivatives of the Theta Nullwert functions have these MacLaurin series: :\theta_{2}'(x) = \frac{\mathrm{d}}{\mathrm{d}x}\,\theta_{2}(x) = \frac{1}{2} x^{-3/4}+\sum_{n = 1}^{\infty} \frac{1}{2}(2n + 1)^2 x^{(2n-1)(2n+3)/4} :\theta_{3}'(x) = \frac{\mathrm{d}}{\mathrm{d}x}\,\theta_{3}(x) = 2+\sum_{n = 1}^{\infty} 2(n + 1)^2 x^{n(n+2)} :\theta_{4}'(x) = \frac{\mathrm{d}}{\mathrm{d}x}\,\theta_{4}(x) = -2+\sum_{n = 1}^{\infty} 2(n + 1)^2 (-1)^{n+1} x^{n(n+2)} The derivatives of theta zero-value functions are as follows: :\theta_{2}'(x) = \frac{\mathrm{d}}{\mathrm{d}x} \,\theta_{2}(x) = \frac{1}{2\pi x} \theta_{2}(x)\theta_{3}(x)^2 E\biggl[\frac{\theta_{2}(x)^2}{\theta_{3}(x)^2}\biggr] :\theta_{3}'(x) = \frac{\mathrm{d}}{\mathrm{d}x} \,\theta_{3}(x) = \theta_{3}(x)\bigl[\theta_{3}(x)^2 + \theta_{4}(x)^2\bigr]\biggl\{\frac{1}{2\pi x}E\biggl[\frac{\theta_{3}(x)^2 - \theta_{4}(x)^2}{\theta_{3}(x)^2 + \theta_{4}(x)^2}\biggr] - \frac{\theta_{4}(x)^2}{4\,x}\biggr\} :\theta_{4}'(x) = \frac{\mathrm{d}}{\mathrm{d}x} \,\theta_{4}(x) = \theta_{4}(x)\bigl[\theta_{3}(x)^2 + \theta_{4}(x)^2\bigr]\biggl\{\frac{1}{2\pi x}E\biggl[\frac{\theta_{3}(x)^2 - \theta _{4}(x)^2}{\theta_{3}(x)^2+\theta_{4}(x)^2}\biggr] - \frac{\theta _{3}(x)^ 2}{4\,x}\biggr\} The two last mentioned formulas are valid for all real numbers of the real definition interval: -1 And these two last named theta derivative functions are related to each other in this way: :\vartheta _{4}(x)\biggl[\frac{\mathrm{d}}{\mathrm{d}x} \,\vartheta _{3}(x)\biggr] - \vartheta _{3}(x)\biggl[\frac{\mathrm{d}}{\mathrm{d}x} \,\theta _{4}(x)\biggr] = \frac{1}{4\,x}\,\theta_{3}(x)\,\theta_{4}(x)\bigl[\theta_{3}(x)^4 - \theta_{4}(x)^4\bigr] The derivatives of the quotients from two of the three theta functions mentioned here always have a rational relationship to those three functions: :\frac{\mathrm{d}}{\mathrm{d}x} \,\frac{\theta _{2}(x)}{\theta _{3}(x)} = \frac{\theta_{2}(x)\,\theta _{4}(x)^4}{4\,x\,\theta _{3}(x)} :\frac{\mathrm{d}}{\mathrm{d}x} \,\frac{\theta _{2}(x)}{\theta _{4}(x)} = \frac{\theta_{2}(x)\,\theta _{3}(x)^4}{4\,x\,\theta _{4}(x)} :\frac{\mathrm{d}}{\mathrm{d}x} \,\frac{\theta _{3}(x)}{\theta _{4}(x)} = \frac{\theta_{3}(x)^5 - \theta _{3}(x)\,\theta _{4}(x)^4}{4\,x\,\theta _{4}(x)} For the derivation of these derivation formulas see the articles Nome (mathematics) and Modular lambda function! == Integrals of theta functions ==

For the theta functions these integrals are valid: :\int_{0}^{1} \theta_{2}(x) \,\mathrm{d}x = \sum _{k = -\infty}^{\infty} \frac{4}{ (2k+1)^2+4} = \pi\tanh(\pi) \approx 3.129881 :\int_{0}^{1} \theta_{3}(x) \,\mathrm{d}x = \sum _{k = -\infty}^{\infty} \frac{1}{ k^2+1} = \pi\coth(\pi) \approx 3.153348 :\int_{0}^{1} \theta_{4}(x) \,\mathrm{d}x = \sum _{k = -\infty}^{\infty} \frac{(-1 )^{k}}{k^2+1} = \pi\,\operatorname{csch}(\pi) \approx 0.272029 The final results now shown are based on the general Cauchy sum formulas. ==A solution to the heat equation==

The Jacobi theta function is the fundamental solution of the one-dimensional heat equation with spatially periodic boundary conditions. Taking to be real and with real and positive, we can write :\vartheta (x;it)=1+2\sum_{n=1}^\infty \exp\left(-\pi n^2 t\right) \cos(2\pi nx) which solves the heat equation :\frac{\partial}{\partial t} \vartheta(x;it)=\frac{1}{4\pi} \frac{\partial^2}{\partial x^2} \vartheta(x;it). This theta-function solution is 1-periodic in , and as it approaches the periodic delta function, or Dirac comb, in the sense of distributions :\lim_{t\to 0} \vartheta(x;it)=\sum_{n=-\infty}^\infty \delta(x-n). General solutions of the spatially periodic initial value problem for the heat equation may be obtained by convolving the initial data at with the theta function. ==Relation to the Heisenberg group==

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==