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{{Short description|Mathematical function}}
In [[mathematics]], the '''Jacobi elliptic functions''' are a set of basic [[elliptic function]]s. They are found in the description of the
==Overview==
[[Image:JacobiFunctionAbstract.png|322px|thumb|The fundamental rectangle in the complex plane of <math>u</math>]]
There are twelve Jacobi elliptic functions denoted by <math>\operatorname{pq}(u, m)</math>, where <math>\mathrm p</math> and <math>\mathrm q</math> are any of the letters <math>\mathrm c</math>, <math>\mathrm s</math>, <math>\mathrm n</math>, and <math>\mathrm d</math>. (Functions of the form <math>\operatorname{pp}(u,m)</math> are trivially set to unity for notational completeness.) <math>u</math> is the argument, and <math>m</math> is the parameter, both of which may be complex. In fact, the Jacobi elliptic functions are [[meromorphic function|meromorphic]] in both <math>u</math> and <math>m</math>.<ref name="Walker">{{cite journal |last1=Walker |first1=Peter |date=2003 |title=The Analyticity of Jacobian Functions with Respect to the Parameter k |url=https://www.jstor.org/stable/3560143 |bibcode=<!-- useless bibcode 2003RSPSA.459.2569W --> |journal=Proceedings of the Royal Society |volume=459 |issue=2038 |pages=2569–2574|doi=10.1098/rspa.2003.1157 |jstor=3560143
In the complex plane of the argument <math>u</math>, the twelve functions form a repeating lattice of simple [[Zeros and poles|poles and zeroes]].<ref name="DLMF22">{{cite web|url=http://dlmf.nist.gov/22|title=NIST Digital Library of Mathematical Functions (Release 1.0.17)|editor-last=Olver|editor-first=F. W. J.|display-editors=et al |date=2017-12-22|publisher=National Institute of Standards and Technology|access-date=2018-02-26 }}</ref> Depending on the function, one repeating [[parallelogram]], or unit cell, will have sides of length <math>2K</math> or <math>4K</math> on the real axis, and <math>2K'</math> or <math>4K'</math> on the imaginary axis, where <math>K=K(m)</math> and <math>K'=K(1-m)</math> are known as the [[quarter period]]s with <math>K(\cdot)</math> being the [[elliptic integral]] of the first kind. The nature of the unit cell can be determined by inspecting the "auxiliary rectangle" (generally a parallelogram), which is a rectangle formed by the origin <math>(0,0)</math> at one corner, and <math>(K,K')</math> as the diagonally opposite corner. As in the diagram, the four corners of the auxiliary rectangle are named <math>\mathrm s</math>, <math>\mathrm c</math>, <math>\mathrm d</math>, and <math>\mathrm n</math>, going counter-clockwise from the origin. The function <math>\operatorname{pq}(u,m)</math> will have a zero at the <math>\mathrm p</math> corner and a pole at the <math>\mathrm q</math> corner. The twelve functions correspond to the twelve ways of arranging these poles and zeroes in the corners of the rectangle.
When the argument <math>u</math> and parameter <math>m</math> are real, with <math>0 < m < 1</math>, <math>K</math> and <math>K'</math> will be real and the auxiliary parallelogram will in fact be a rectangle, and the Jacobi elliptic functions will all be real valued on the real line.
Since the
The
* There is a simple zero at the corner <math>\mathrm p</math>, and a simple pole at the corner <math>\mathrm q</math>.
* The complex number <math>\mathrm p-\mathrm q</math> is equal to half the period of the function <math>\operatorname{pq} u</math>; that is, the function <math>\operatorname{pq} u</math> is periodic in the direction <math>\operatorname{pq}</math>, with the period being <math>2(\mathrm p-\mathrm q)</math>. The function <math>\operatorname{pq} u</math> is also periodic in the other two directions <math>\mathrm{pp}'</math> and <math>\mathrm{pq}'</math>, with periods such that <math>\mathrm p-\mathrm p'</math> and <math>\mathrm p-\mathrm q'</math> are quarter periods.
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[[File:Modell der elliptischen Funktion φ=am (u, k) durch eine Fläche -Schilling V, 1 - 317-.jpg|thumb|Model of the Jacobi amplitude (measured along vertical axis) as a function of independent variables ''u'' and the modulus ''k'']]
There is a definition, relating the elliptic functions to the inverse of the
:<math>u=F(\varphi,m)=\int_0^\varphi \frac{\mathrm d\theta} {\sqrt {1-m \sin^2 \theta}}</math>
is called the '''Jacobi amplitude''':
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:<math>\operatorname {dn} (u,m) = \frac{\mathrm d}{\mathrm du}\operatorname{am}(u,m).</math>
In the above, the value <math>m</math> is a free parameter, usually taken to be real such that <math>0\leq m \leq 1</math> (but can be complex in general), and so the elliptic functions can be thought of as being given by two variables, <math>u</math> and the parameter <math>m</math>. The remaining nine elliptic functions are easily built from the above three (<math>\operatorname{sn}</math>, <math>\operatorname{cn}</math>, <math>\operatorname{dn}</math>), and are given in a section below. Note that when <math>\varphi=\pi/2</math>, that <math>u</math> then equals the [[quarter period]] <math>K</math>.
In the most general setting, <math>\operatorname{am}(u,m)</math> is a [[multivalued function]] (in <math>u</math>) with infinitely many [[Branch point|logarithmic branch points]] (the branches differ by integer multiples of <math>2\pi</math>), namely the points <math>2sK(m)+(4t+1)K(1-m)i</math> and <math>2sK(m)+(4t+3)K(1-m)i</math> where <math>s,t\in\mathbb{Z}</math>.<ref name="sala">{{cite journal |last=Sala |first=Kenneth L. |date=November 1989 |title=Transformations of the Jacobian Amplitude Function and Its Calculation via the Arithmetic-Geometric Mean
However, a particular cutting for <math>\operatorname{am}(u,m)</math> can be made in the
But defining <math>\operatorname{am}(u,m)</math> this way gives rise to very complicated branch cuts in the <math>m</math>-plane (''not'' the <math>u</math>-plane); they have not been fully described as of yet.
Let
:<math>
Then the '''Jacobi epsilon''' function can be defined as
:<math>\mathcal{E}(u,m)=E(\operatorname{am}(u,m),m)</math>
for <math>u\in\mathbb{R}</math> and <math>0<m<1</math> and by [[analytic continuation]] in each of the variables otherwise: the Jacobi epsilon function is meromorphic in the whole complex plane (in both <math>u</math> and <math>m</math>). Alternatively, throughout both the <math>u</math>-plane and <math>m</math>-plane,<ref>{{dlmf|first1=W. P.|last1=Reinhardt|first2=P. L.|last2=Walker|id=22.16.E17|title=Jacobian Elliptic Functions}}</ref>
:<math>\mathcal{E} (u,m)=\int_0^u \operatorname{dn}^2(t,m)\, \mathrm dt;</math>
<math>\mathcal{E}</math> is well-defined in this way because all [[Residue (complex analysis)|residues]] of <math>t\mapsto\operatorname{dn}(t,m)^2</math> are zero, so the integral is path-independent. So the Jacobi epsilon relates the incomplete elliptic integral of the first kind to the incomplete elliptic integral of the second kind:
:<math>E(\varphi,m)=\mathcal{E}(F(\varphi,m),m).</math>
The Jacobi epsilon function is not an elliptic function
The '''Jacobi zn''' function is defined by
:<math>\operatorname{zn}(u,m)=\
It is a singly periodic function which is meromorphic in <math>u</math>, but not in <math>m</math> (due to the branch cuts of <math>E</math> and <math>K</math>). Its minimal period in <math>u</math> is <math>2K(m)</math>. It is related to the [[Elliptic integrals#Jacobi zeta function|Jacobi zeta function]] by <math>Z(\varphi,m)=\operatorname{zn}(F(\varphi,m),m).</math>
Historically, the Jacobi elliptic functions were first defined by using the amplitude. In more modern texts on elliptic functions, the Jacobi elliptic functions are defined by other means, for example by ratios of theta functions (see below), and the amplitude is ignored.
In modern terms, the relation to elliptic integrals would be expressed by <math>\operatorname{sn}(F(\varphi,m),m)=\sin\varphi</math> (or <math>\operatorname{cn}(F(\varphi,m),m)=\cos\varphi</math>) instead of <math>\operatorname{am}(F(\varphi,m),m)=\varphi</math>.
==Definition as trigonometry: the Jacobi ellipse==
[[File:Jacobi Elliptic Functions (on Jacobi Ellipse).svg|right|thumb|upright=1.5|Plot of the Jacobi ellipse (''x''<sup>2</sup> + ''y''<sup>2</sup>/''b''<sup>2</sup> = 1, ''b'' real) and the twelve Jacobi elliptic functions ''pq''(''u'',''m'') for particular values of angle ''φ'' and parameter ''b''. The solid curve is the ellipse, with ''m'' = 1 − 1/''b''<sup>2</sup> and ''u'' = ''F''(''φ'',''m'') where ''F''(⋅,⋅) is the [[elliptic integral]] of the first kind (with parameter <math>m=k^2</math>). The dotted curve is the unit circle. Tangent lines from the circle and ellipse at ''x'' = cd crossing the ''x''-axis at dc are shown in light grey.]]
<math> \cos \varphi, \, \sin \varphi </math> are defined on the unit circle
:<math>
\begin{align}
& x^2 + \frac{y^2}{b^2} = 1, \quad m
&
\end{align}
</math>
Then <math>0 \le m < 1</math> and
:<math> r(
For each angle <math>\varphi</math> the parameter
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(the incomplete elliptic integral of the first kind) is computed.
On the unit circle (<math>a=b=1</math>), <math>u</math> would be an arc length.
Then the familiar relations from the unit circle
:<math> x' = \cos \varphi, \quad y' = \sin \varphi</math>
read for the ellipse
:<math>x' = \operatorname{cn}(u,m),\quad y' = \operatorname{sn}(u,m).</math>
So the projections of the intersection point <math>P'</math> of the line <math>OP</math> with the unit circle on the ''x''- and ''y''-axes are simply <math>\operatorname{cn}(u,m)</math> and <math>\operatorname{sn}(u,m)</math>. These projections may be interpreted as 'definition as trigonometry'. In short,
:<math> \operatorname{cn}(u,m) = \frac{x}{r(\varphi,m)}, \quad \operatorname{sn}(u,m) = \frac{y}{r(\varphi,m)}, \quad \operatorname{dn}(u,m) = \frac{1}{r(\varphi,m)}. </math>
For the <math>x</math> and <math>y</math>
<math>u</math> and parameter <math>m</math> we get,
:<math>r(\varphi,m) = \frac 1 {\operatorname{dn}(u,m)} </math>
into <math>x = r(\varphi,m) \cos \varphi</math> and <math>y = r(\varphi,m) \sin \varphi</math>, we find
:<math> x = \frac{\operatorname{cn}(u,m)} {\operatorname{dn}(u,m)},\quad y = \frac{\operatorname{sn}(u,m)} {\operatorname{dn}(u,m)}.</math>
The latter relations for the ''x''- and ''y''-coordinates of points on the unit ellipse may be considered as generalization of the relations <math> x = \cos \varphi</math> and <math>y = \sin \varphi</math> for the coordinates of points on the unit circle.
The following table summarizes the expressions for all Jacobi elliptic functions pq(u,m) in the variables (''x'',''y'',''r'') and (''φ'',dn) with <math display="inline">r = \sqrt{x^2+y^2}</math>.
{| class="wikitable" style="text-align:center"
|+ Jacobi elliptic functions pq[''u'',''m''] as functions of {''x'',''y'',''r''} and {''φ'',dn}
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|}
==Definition in terms of the Jacobi theta functions==
===Using elliptic integrals===
Equivalently, Jacobi's elliptic functions can be defined in terms of the [[theta function]]s.<ref>{{cite book |last1=Whittaker |first1=Edmund Taylor |authorlink1=Edmund T. Whittaker |last2=Watson |first2=George Neville |authorlink2=George N. Watson |date= 1927 |page=492 |edition=4th |title=A Course of Modern Analysis |title-link=A Course of Modern Analysis |publisher= Cambridge University Press}}</ref> With <math>z,\tau\in\mathbb{C}</math> such that <math>\operatorname{Im}\tau >0</math>, let
:<math>\theta_1(z|\tau)=\displaystyle\sum_{n=-\infty}^\infty (-1)^{n-\frac12}e^{(2n+1)iz+\pi i\tau\left(n+\frac12\right)^2},</math>
:<math>\theta_2(z|\tau)=\displaystyle\sum_{n=-\infty}^\infty e^{(2n+1)iz+\pi i\tau \left(n+\frac12\right)^2},</math>
:<math>\theta_3(z|\tau)=\displaystyle\sum_{n=-\infty}^\infty e^{2niz+\pi i\tau n^2},</math>
:<math>\theta_4(z|\tau)=\displaystyle\sum_{n=-\infty}^\infty (-1)^n e^{2niz+\pi i\tau n^2}</math>
and let <math>\theta_2(\tau)=\theta_2(0|\tau)</math>, <math>\theta_3(\tau)=\theta_3(0|\tau)</math>, <math>\theta_4(\tau)=\theta_4(0|\tau)</math>. Then with <math>K=K(m)</math>, <math>K'=K(1-m)</math>, <math>\zeta=\pi u/(2K)</math> and <math>\tau=iK'/K</math>,
:<math>\begin{align}\operatorname{sn}(u,m)&=\frac{\theta_3(\tau)\theta_1(\zeta|\tau)}{\theta_2(\tau)\theta_4(\zeta|\tau)},\\
\operatorname{cn}(u,m)&=\frac{\theta_4(\tau)\theta_2(\zeta|\tau)}{\theta_2(\tau)\theta_4(\zeta|\tau)},\\
\operatorname{
The Jacobi zn function can be expressed by theta functions as well:
:<math>\begin{align}\operatorname{zn}(u
&=\frac{\pi}{2K}\frac{\
&=\frac{\pi}{2K}\frac{\
where <math>'</math> denotes the [[partial derivative]] with respect to the first variable.
===Using modular inversion===
In fact, the definition of the Jacobi elliptic functions in Whittaker & Watson is stated a little bit differently than the one given above (but it's equivalent to it) and relies on modular inversion: [[Modular lambda function|The function]] <math>\lambda</math>, defined by
[[File:The region F1 for modular inversion.jpg|thumb|The region <math>F_1</math> in the complex plane. It is bounded by two semicircles from below, by a ray from the left and by a ray from the right.]]
:<math>\lambda (\tau)=\frac{\theta_2(\tau)^4}{\theta_3(\tau)^4},</math>
assumes every value in <math>\mathbb{C}-\{0,1\}</math> ''once and only once''<ref>{{cite journal |last=Cox |first=David Archibald |authorlink1=David A. Cox |date=January 1984 |title=The Arithmetic-Geometric Mean of Gauss|url=https://www.researchgate.net/publication/248675540 |journal=L'Enseignement Mathématique|volume=30|issue=2|pages=290}}</ref> in
:<math>F_1-(\partial F_1\cap\{\tau\in\mathbb{H}:\operatorname{Re}\tau <0\})</math>
where <math>\mathbb{H}</math> is the upper half-plane in the complex plane, <math>\partial F_1</math> is the boundary of <math>F_1</math> and
:<math>F_1=\{\tau\in\mathbb{H}:\left|\operatorname{Re}\tau\right|\le 1,\left|\operatorname{Re}(1/\tau)\right|\le 1\}.</math>
In this way, each <math>m\,\overset{\text{def}}{=}\,\lambda (\tau)\in\mathbb{C}-\{0,1\}</math> can be associated with ''one and only one'' <math>\tau</math>. Then Whittaker & Watson define the Jacobi elliptic functions by
:<math>\begin{align}\operatorname{sn}(u,m)&=\frac{\theta_3(\tau)\theta_1(\zeta |\tau)}{\theta_2(\tau)\theta_4(\zeta|\tau)},\\
\operatorname{cn}(u,m)&=\frac{\theta_4(\tau)\theta_2(\zeta |\tau)}{\theta_2(\tau)\theta_4(\zeta|\tau)},\\
\operatorname{dn}(u,m)&=\frac{\theta_4(\tau)\theta_3(\zeta |\tau)}{\theta_3(\tau)\theta_4(\zeta|\tau)}\end{align}</math>
where <math>\zeta=u/\theta_3(\tau)^2</math>.
In the book, they place an additional restriction on <math>m</math> (that <math>m\notin (-\infty,0)\cup (1,\infty)</math>), but it is in fact not a necessary restriction (see the Cox reference). Also, if <math>m=0</math> or <math>m=1</math>, the Jacobi elliptic functions degenerate to non-elliptic functions which is described below.
==Definition in terms of Neville theta functions==
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|}
The value of the Jacobi transformations is that any set of Jacobi elliptic functions with any
===Amplitude transformations===
In the following, the second variable is suppressed and is equal to <math>m</math>:
:<math>\sin(\operatorname{am}(u+v)+\operatorname{am}(u-v))=\frac{2\operatorname{sn}u\operatorname{cn}u\operatorname{dn}v}{1-m\operatorname{sn}^2u\operatorname{sn}^2v},</math>
:<math>\cos(\operatorname{am}(u+v)-\operatorname{am}(u-v))=\dfrac{\operatorname{cn}^2v-\operatorname{sn}^2v\operatorname{dn}^2u}{1-m\operatorname{sn}^2u\operatorname{sn}^2v}</math>
where both identities are valid for all <math>u,v,m\in\mathbb{C}</math> such that both sides are well-defined.
With
:<math>m_1=\left(\frac{1-\sqrt{m'}}{1+\sqrt{m'}}\right)^2,</math>
we have
:<math>\cos (\operatorname{am}(u,m)+\operatorname{am}(K-u,m))=-\operatorname{sn}((1-\sqrt{m'})u,1/m_1),</math>
:<math>\sin(\operatorname{am}(\sqrt{m'}u,-m/m')+\operatorname{am}((1-\sqrt{m'})u,1/m_1))=\operatorname{sn}(u,m),</math>
:<math>\sin(\operatorname{am}((1+\sqrt{m'})u,m_1)+\operatorname{am}((1-\sqrt{m'})u,1/m_1))=\sin(2\operatorname{am}(u,m))</math>
where all the identities are valid for all <math>u,m\in\mathbb{C}</math> such that both sides are well-defined.
==The Jacobi hyperbola==
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In the complex plane of the argument ''u'', the Jacobi elliptic functions form a repeating pattern of poles (and zeroes). The residues of the poles all have the same absolute value, differing only in sign. Each function pq(''u'',''m'') has an "inverse function" (in the multiplicative sense) qp(''u'',''m'') in which the positions of the poles and zeroes are exchanged. The periods of repetition are generally different in the real and imaginary directions, hence the use of the term "doubly periodic" to describe them.
:<math>\operatorname{am}(u+2K,m)=\operatorname{am}(u,m)+\pi,</math>
:<math>\
:<math>\mathcal{E}(u+2K,m)=\mathcal{E}(u,m)+2E,</math>
:<math>\mathcal{E}(u+2iK',m)=\mathcal{E}(u,m)+2iE \frac{K'}{K}-\frac{\pi i}{K}</math>
where <math>E(m)</math> is the [[Elliptic integral#Complete elliptic integral of the second kind|complete elliptic integral of the second kind]] with parameter <math>m</math>.
The double periodicity of the Jacobi elliptic functions may be expressed as:
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When applicable, poles displaced above by 2''K'' or displaced to the right by 2''K''′ have the same value but with signs reversed, while those diagonally opposite have the same value. Note that poles and zeroes on the left and lower edges are considered part of the unit cell, while those on the upper and right edges are not.
The information about poles can in fact be used to [[Characterization (mathematics)|characterize]] the Jacobi elliptic functions:<ref>{{cite book |last1=Whittaker |first1=Edmund Taylor |authorlink1=Edmund T. Whittaker |last2=Watson |first2=George Neville |authorlink2=George N. Watson |date= 1927 |pages=504–505 |edition=4th |title=A Course of Modern Analysis |title-link=A Course of Modern Analysis |publisher= Cambridge University Press}}</ref>
The function <math>u\mapsto\operatorname{sn}(u,m)</math> is the unique elliptic function having simple poles at <math>2rK+(2s+1)iK'</math> (with <math>r,s\in\mathbb{Z}</math>) with residues <math>(-1)^r/\sqrt{m}</math> taking the value <math>0</math> at <math>0</math>.
The function <math>u\mapsto\operatorname{cn}(u,m)</math> is the unique elliptic function having simple poles at <math>2rK+(2s+1)iK'</math> (with <math>r,s\in\mathbb{Z}</math>) with residues <math>(-1)^{r+s-1}i/\sqrt{m}</math> taking the value <math>1</math> at <math>0</math>.
The function <math>u\mapsto\operatorname{dn}(u,m)</math> is the unique elliptic function having simple poles at <math>2rK+(2s+1)iK'</math> (with <math>r,s\in\mathbb{Z}</math>) with residues <math>(-1)^{s-1}i</math> taking the value <math>1</math> at <math>0</math>.
==Special values==
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==Identities==
===Half
<math display="block">\operatorname{sn}\left(\frac{u}{2},m\right)=\pm\sqrt{\frac{1-\operatorname{cn}(u,m)}{1+\operatorname{dn}(u,m)}}</math>
<math display="block">\operatorname{cn}\left(\frac{u}{2},m\right)=\pm\sqrt{\frac{\operatorname{cn}(u,m)+\operatorname{dn}(u,m)}{1+\operatorname{dn}(u,m)}}</math>
<math display="block">\operatorname{
===K formulas===
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==Jacobi elliptic functions as solutions of nonlinear ordinary differential equations==
===Derivatives with respect to the first variable===
The [[derivative]]s of the three basic Jacobi elliptic functions (with respect to the first variable, with <math>m</math> fixed) are:
<math display=block>\frac{\mathrm{d}}{\mathrm{d}z} \operatorname{sn}(z) = \operatorname{cn}(z) \operatorname{dn}(z),</math>
<math display=block>\frac{\mathrm{d}}{\mathrm{d}z} \operatorname{cn}(z) = -\operatorname{sn}(z) \operatorname{dn}(z),</math>
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|m' nc sc || −cs ns || −''m'' cn sn ||0
|}
Also
:<math>\frac{\mathrm d}{\mathrm dz}\mathcal{E}(z)=\operatorname{dn}(z)^2.</math>
With the [[#Addition theorems|addition theorems above]] and for a given ''m'' with 0 < ''m'' < 1 the major functions are therefore solutions to the following nonlinear [[ordinary differential equation]]s:
* <math>\operatorname{am}(x)</math> solves the differential equations <math>\frac{\mathrm d^2y}{\mathrm dx^2}+m\sin (y)\cos (y)=0</math> and
:<math>\left(\frac{\mathrm dy}{\mathrm dx}\right)^2=1-m\sin(y)^2</math> (for <math>x</math> not on a branch cut)
* <math>\operatorname{sn}(x)</math> solves the differential equations <math>\frac{\mathrm{d}^2 y}{\mathrm{d}x^2} + (1+m) y - 2 m y^3 = 0</math> and <math> \left(\frac{\mathrm{d} y}{\mathrm{d}x}\right)^2 = (1-y^2) (1-m y^2)</math>
* <math>\operatorname{cn}(x)</math> solves the differential equations <math>\frac{\mathrm{d}^2 y}{\mathrm{d}x^2} + (1-2m) y + 2 m y^3 = 0</math> and <math> \left(\frac{\mathrm{d} y}{\mathrm{d}x}\right)^2 = (1-y^2) (1-m + my^2)</math>
* <math>\operatorname{dn}(x)</math> solves the differential equations <math>\frac{\mathrm{d}^2 y}{\mathrm{d}x^2} - (2 - m) y + 2 y^3 = 0</math> and <math> \left(\frac{\mathrm{d} y}{\mathrm{d}x}\right)^2 = (y^2 - 1) (1 - m - y^2)</math>
The
:<math>\frac{\mathrm d^2 \theta}{\mathrm dt^2}+c\sin \theta=0,</math>
with initial angle <math>\theta_0</math> and zero initial angular velocity is
:<math>\begin{align}\theta
&=2\operatorname{am}\left(\frac{1+\sqrt{m}}{2}(\sqrt{c}t+K),\frac{4\sqrt{m}}{(1+\sqrt{m})^2}\right)-2\operatorname{am}\left(\frac{1+\sqrt{m}}{2}(\sqrt{c}t-K),\frac{4\sqrt{m}}{(1+\sqrt{m})^2}\right)-\pi\end{align}</math>
where <math>m=\sin (\theta_0/2)^2</math>, <math>c>0</math> and <math>t\in\mathbb{R}</math>.
===Derivatives with respect to the second variable===
With the first argument <math>z</math> fixed, the derivatives with respect to the second variable <math>m</math> are as follows:
:<math>\begin{align}\frac{\mathrm d}{\mathrm dm}\operatorname{sn}(z)&=\frac{\operatorname{dn}(z)\operatorname{cn}(z)((1-m)z-\mathcal{E}(z)+m\operatorname{cd}(z)\operatorname{sn}(z))}{2m(1-m)},\\
\frac{\mathrm d}{\mathrm dm}\operatorname{cn}(z)&=\frac{\operatorname{sn}(z)\operatorname{dn}(z)((m-1)z+\mathcal{E}(z)-m\operatorname{sn}(z)\operatorname{cd}(z))}{2m(1-m)},\\
\frac{\mathrm d}{\mathrm dm}\operatorname{dn}(z)&=\frac{\operatorname{sn}(z)\operatorname{cn}(z)((m-1)z+\mathcal{E}(z)-\operatorname{dn}(z)\operatorname{sc}(z))}{2(1-m)},\\
\frac{\mathrm d}{\mathrm dm}\mathcal{E}(z)&=\frac{\operatorname{cn}(z)(\operatorname{sn}(z)\operatorname{dn}(z)-\operatorname{cn}(z)\mathcal{E}(z))}{2(1-m)}-\frac{z}{2}\operatorname{sn}(z)^2.\end{align}</math>
==Expansion in terms of the nome==
Let the [[nome (mathematics)|nome]] be <math>q=\exp(-\pi K'(m)/K(m))=e^{i\pi \tau}</math>, <math>\operatorname{Im}(\tau)>0</math>, <math>m=k^2</math> and let <math>v=\pi u /(2K(m))</math>. Then the functions have expansions as [[Lambert series]]
:<math>\operatorname{am}(u,m)=\frac{\pi u}{2K(m)}+2\sum_{n=1}^\infty \frac{q^n}{n(1+q^{2n})}\sin (2nv),</math>
:<math>\operatorname{sn}(u,m)=\frac{2\pi}{kK(m)}
Line 739 ⟶ 777:
:<math>\operatorname{dn}(u,m)=\frac{\pi}{2K(m)} + \frac{2\pi}{K(m)}
\sum_{n=1}^\infty \frac{q^{n}}{1+q^{2n}} \cos (2nv),</math>
:<math>\operatorname{zn}(u,m)=\frac{2\pi}{K(m)}\sum_{n=1}^\infty \frac{q^n}{1-q^{2n}}\sin (2nv)</math>
when <math>
Bivariate power series expansions have been published by Schett.<ref>{{cite journal|first1=Alois|last1=Schett |title=Properties of the Taylor series expansion coefficients of the Jacobian Elliptic Functions|year=1976|journal=Math. Comp.|volume=30|number=133|pages=143–147|doi=10.1090/S0025-5718-1976-0391477-3| mr=0391477 |doi-access=free }}</ref>
==Fast computation==
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Initialize
:<math>a_0=1,\, b_0=\sqrt{1-m
where <math>0<m<1</math>.
Define
Line 767 ⟶ 799:
:<math>\varphi_{n-1}=\frac{1}{2}\left(\varphi_n+\arcsin \left(\frac{c_n}{a_n}\sin \varphi_n\right)\right)</math>
for <math>n\ge 1</math>, then
:<math>\operatorname{am}(u,m)=\varphi_0,\quad \operatorname{zn}(u,m)=\sum_{n=1}^N c_n\sin\varphi_n</math>
as <math>N\to\infty</math>. This is notable for its rapid convergence. It is then trivial to compute all Jacobi elliptic functions from the Jacobi amplitude <math>\operatorname{am}</math> on the real line.<ref group="note">For the <math>\operatorname{dn}</math> function,
<math>\operatorname{dn}(u,m)=\frac{\operatorname{cn}(u,m)}{\operatorname{sn}(K(m)-u,m)}</math> can be used.</ref>
In conjunction with the addition theorems for elliptic functions (which hold for complex numbers in general) and the Jacobi transformations, the method of computation described above can be used to compute all Jacobi elliptic functions in the whole complex plane.
Another method of fast computation of the Jacobi elliptic functions via the arithmetic–geometric mean, avoiding the computation of the Jacobi amplitude, is due to Herbert E. Salzer:<ref>{{cite journal |last=Salzer |first=Herbert E. |date=July 1962 |title=Quick calculation of Jacobian elliptic functions|journal=Communications of the ACM|volume=5|issue=7|pages=399|doi=10.1145/368273.368573
Let
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\operatorname{dn}(u,m)&=\sqrt{1-\frac{m}{y_0^2}}\end{align}</math>
as <math>N\to\infty</math>.
Yet, another method for a rapidly converging fast computation of the Jacobi elliptic sine function found in the literature is shown below.<ref>{{Cite journal |last=Smith |first=John I. |date=May 5, 1971 |title=The Even- and Odd-Mode Capacitance Parameters for Coupled Lines in Suspended Substrate |journal=IEEE Transactions on Microwave Theory and Techniques |volume=MTT-19 |issue=5 |pages=430 |doi=10.1109/TMTT.1971.1127543 |bibcode=<!-- useless bibcode 1971ITMTT..19..424S --> }}</ref>
Let:
:<math>\begin{align}
&a_0 = u &b_0 = \frac{1-\sqrt{1-m}}{1+\sqrt{1-m}} \\
&a_1 = \frac{a_0}{1+b_0} &b_1 = \frac{1-\sqrt{1-b^2_0 }}{1+\sqrt{1-b^2_0}}\\
&\vdots = \vdots &\vdots = \vdots \\
&a_n = \frac{a_{n-1}}{1+b_{n-1}} &b_n = \frac{1-\sqrt{1-b^2_{n-1}}}{1+\sqrt{1-b^2_{n-1}}}\\
\end{align}</math>
Then set:
:<math>\begin{align}
y_{n+1} &= \sin(a_n) \\
y_{n} &= \frac{y_{n+1}(1+b_n)}{1+y^2_{n+1}b_n} \\
\vdots &= \vdots\\
y_0 &= \frac{y_1(1+b_0)}{1+y^2_1b_0} \\
\end{align}</math>
Then:
:<math>\operatorname{sn}(u,m) = y_0 \text{ as }n \rightarrow\infty</math>.
==Approximation in terms of hyperbolic functions==
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==Continued fractions==
Assuming real numbers <math>a,p</math> with <math>0<a<p</math> and the [[Nome (mathematics)|nome]] <math>q=e^{\pi i \tau}</math>, <math>\operatorname{Im}(\tau)>0</math> with [[Theta function|elliptic modulus]] <math display="inline">k(\tau)=\sqrt{1-k'(\tau)^2}=(\vartheta_{10}(0;\tau)/\vartheta_{00}(0;\tau))^2</math>. If <math>K[\tau]=K(k(\tau))</math>, where <math>K(x)=\pi/2\cdot {}_2F_1(1/2,1/2;1;x^2)</math> is the [[complete elliptic integral of the first kind]], then holds the following [[Continued fraction|continued fraction expansion]]<ref name="bagis-evaluations">{{citation |mode=cs1 |type=Preprint |first=N. |last=Bagis
:<math>
\begin{align}
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== {{anchor|arcsn|arccn|arcdn}}Inverse functions ==
The inverses of the Jacobi elliptic functions can be defined similarly to the [[inverse trigonometric functions]]; if <math>x=\operatorname{sn}(\xi, m)</math>, <math>\xi=\operatorname{arcsn}(x, m)</math>. They can be represented as elliptic integrals,<ref>{{dlmf|title=§22.15 Inverse Functions|first1=W. P.|last1=Reinhardt|first2=P. L.|last2=Walker|id=22.15}}</ref><ref>{{cite web|last=Ehrhardt|first=Wolfgang|title=The AMath and DAMath Special Functions: Reference Manual and Implementation Notes|url=http://www.wolfgang-ehrhardt.de/specialfunctions.pdf|access-date=17 July 2013|page=42|archive-url=https://web.archive.org/web/20160731033441/http://www.wolfgang-ehrhardt.de/specialfunctions.pdf|archive-date=31 July 2016|url-status=dead}}</ref><ref>{{cite book|last1=Byrd|first1=P.F.|last2=Friedman|first2=M.D.|title=Handbook of Elliptic Integrals for Engineers and Scientists|date=1971|publisher=Springer-Verlag|___location=Berlin|edition=2nd}}</ref> and power series representations have been found.<ref>{{cite journal|last=Carlson|first=B. C.|title=Power series for inverse Jacobian elliptic functions|journal=Mathematics of Computation|year=2008|volume=77|issue=263|pages=1615–1621|url=https://www.ams.org/journals/mcom/2008-77-263/S0025-5718-07-02049-2/S0025-5718-07-02049-2.pdf|access-date=17 July 2013|doi=10.1090/s0025-5718-07-02049-2|bibcode=<!-- useless bibcode 2008MaCom..77.1615C -->|doi-access=free}}</ref><ref name="DLMF22"/>
*<math>\operatorname{arcsn}(x,m) = \int_0^x \frac{\mathrm{d}t}{\sqrt{(1-t^2)(1-mt^2)}}</math>
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* [[Dixon elliptic functions]]
* [[Abel elliptic functions]]
* [[Weierstrass elliptic
* [[Lemniscate elliptic functions]]
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