# Weierstrass's elliptic functions

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In mathematics, Weierstrass's elliptic functions are elliptic functions that take a particularly simple form. They are named for Karl Weierstrass. This class of functions are also referred to as p-functions and they are usually denoted by the symbol ℘. They play an important role in theory of elliptic functions. A ℘-function together with its derivative can be used to parameterize elliptic curves and they generate the field of elliptic functions with respect to a given period lattice.

Symbol for Weierstrass ${\displaystyle \wp }$-function

Model of Weierstrass ${\displaystyle \wp }$-function

## Definition

Visualization of the ${\displaystyle \wp }$-function with invariants ${\displaystyle g_{2}=1+i}$ and ${\displaystyle g_{3}=2-3i}$ in which white corresponds to a pole, black to a zero.

Let ${\displaystyle \omega _{1},\omega _{2}\in \mathbb {C} }$ be two complex numbers that are linear independent over ${\displaystyle \mathbb {R} }$ and let ${\displaystyle \Lambda :=\mathbb {Z} \omega _{1}+\mathbb {Z} \omega _{2}:=\{m\omega _{1}+n\omega _{2}:m,n\in \mathbb {Z} \}}$ be the lattice generated by those numbers. Then the ${\displaystyle \wp }$-function is defined as follows:

${\displaystyle \wp (z,\omega _{1},\omega _{2}):=\wp (z,\Lambda ):={\frac {1}{z^{2}}}+\sum _{\lambda \in \Lambda \setminus \{0\}}\left({\frac {1}{(z-\lambda )^{2}}}-{\frac {1}{\lambda ^{2}}}\right).}$

This series converges locally uniformly absolutely in ${\displaystyle \mathbb {C} \setminus \Lambda }$. Oftentimes instead of ${\displaystyle \wp (z,\omega _{1},\omega _{2})}$ only ${\displaystyle \wp (z)}$ is written.

The Weierstrass ${\displaystyle \wp }$-function is constructed exactly in such a way that it has a pole of the order two at each lattice point.

Because the sum ${\textstyle \sum _{\lambda \in \Lambda }{\frac {1}{(z-\lambda )^{2}}}}$ alone would not converge it is necessary to add the term ${\textstyle -{\frac {1}{\lambda ^{2}}}}$.[1]

It is common to use ${\displaystyle 1}$ and ${\displaystyle \tau \in \mathbb {H} :=\{z\in \mathbb {C} :\operatorname {Im} (z)>0\}}$ as generators of the lattice. Multiplying by ${\textstyle {\frac {1}{\omega _{1}}}}$ maps the lattice ${\displaystyle \mathbb {Z} \omega _{1}+\mathbb {Z} \omega _{2}}$ isomorphically onto the lattice ${\displaystyle \mathbb {Z} +\mathbb {Z} \tau }$ with ${\textstyle \tau ={\frac {\omega _{2}}{\omega _{1}}}}$. By possibly substituting ${\displaystyle \tau }$ by ${\displaystyle -\tau }$ it can be assumed that ${\displaystyle \tau \in \mathbb {H} }$. One sets ${\displaystyle \wp (z,\tau ):=\wp (z,1,\tau )}$.

## Motivation

A cubic of the form ${\displaystyle C_{g_{2},g_{3}}^{\mathbb {C} }=\{(x,y)\in \mathbb {C} ^{2}:y^{2}=4x^{3}-g_{2}x+g_{3}\}}$, where ${\displaystyle g_{2},g_{3}\in \mathbb {C} }$ are complex numbers with ${\displaystyle g_{2}^{3}-27g_{3}^{2}\neq 0}$, can not be rationally parameterized.[2] Yet one still wants to find a way to parameterize it.

For the quadric ${\displaystyle K=\{(x,y)\in \mathbb {R} ^{2}:x^{2}+y^{2}=1\}}$, the unit circle, there exists a (non-rational) parameterization using the sine function and its derivative the cosine function:

${\displaystyle \psi :\mathbb {R} /2\pi \mathbb {Z} \to K,\quad t\mapsto (\sin(t),\cos(t))}$.

Because of the periodicity of the sine and cosine ${\displaystyle \mathbb {R} /2\pi \mathbb {Z} }$ is chosen to be the domain, so the function is bijective.

In a similar way one can get a parameterization of ${\displaystyle C_{g_{2},g_{3}}^{\mathbb {C} }}$ by means of the doubly periodic ${\displaystyle \wp }$-function (see in the section "Relation to ellitpic curves"). This parameterization has the domain ${\displaystyle \mathbb {C} /\Lambda }$, which is topologically equivalent to a torus.[3]

There is another analogy to the trigonometric functions. Consider the integral function

${\displaystyle a(x)=\int _{0}^{x}{\frac {dy}{\sqrt {(1-y^{2})}}}}$.

It can be simplified by substituting ${\displaystyle y=\sin(t)}$ and ${\displaystyle s=\arcsin(x)}$:

${\displaystyle a(x)=\int _{0}^{s}dt=s=\arcsin(x)}$.

That means ${\displaystyle a^{-1}(x)=\sin(x)}$. So the sine function is an inverse function of an integral function.[4]

Elliptic functions are also inverse functions of integral functions, namely of elliptic integrals. In particular the ${\displaystyle \wp }$-function is obtained in the following way:

Let

${\displaystyle u(z)=-\int _{z}^{\infty }{\frac {ds}{\sqrt {4s^{3}-g_{2}s-g_{3}}}}}$.

Then ${\displaystyle u^{-1}}$ can be extended to the complex plane and this extension equals the ${\displaystyle \wp }$-function.[5]

## Properties

• ℘ is an even function. That means ${\displaystyle \wp (z)=\wp (-z)}$ for all ${\displaystyle z\in \mathbb {C} \setminus \Lambda }$, which can be seen in the following way:
${\displaystyle \wp (-z)={\frac {1}{(-z)^{2}}}+\sum _{\lambda \in \Lambda \setminus \{0\}}\left({\frac {1}{(-z-\lambda )^{2}}}-{\frac {1}{\lambda ^{2}}}\right)={\frac {1}{z^{2}}}+\sum _{\lambda \in \Lambda \setminus \{0\}}\left({\frac {1}{(z+\lambda )^{2}}}-{\frac {1}{\lambda ^{2}}}\right)={\frac {1}{z^{2}}}+\sum _{\lambda \in \Lambda \setminus \{0\}}\left({\frac {1}{(z-\lambda )^{2}}}-{\frac {1}{\lambda ^{2}}}\right)=\wp (z)}$

The second last equality holds because ${\displaystyle \{-\lambda :\lambda \in \Lambda \}=\Lambda }$. Since the sum converges absolutely this rearrangement does not change the limit.

• ℘ is meromorphic and its derivative is[6]
${\displaystyle \wp '(z)=-2\sum _{\lambda \in \Lambda }{\frac {1}{(z-\lambda )^{3}}}}$.
• ${\displaystyle \wp }$ and ${\displaystyle \wp '}$ are doubly periodic with the periods ${\displaystyle \omega _{1}}$und ${\displaystyle \omega _{2}}$.[6] This means:
${\displaystyle \wp (z+\omega _{1})=\wp (z)=\wp (z+\omega _{2})}$ and ${\displaystyle \wp '(z+\omega _{1})=\wp '(z)=\wp '(z+\omega _{2})}$.

It follows that ${\displaystyle \wp (z+\lambda )=\wp (z)}$ and ${\displaystyle \wp '(z+\lambda )=\wp '(z)}$ for all ${\displaystyle \lambda \in \Lambda }$. Functions which are meromorphic and doubly periodic are also called elliptic functions.

## Laurent expansion

Let ${\displaystyle r:=\min\{{|\lambda }|:0\neq \lambda \in \Lambda \}}$. Then for ${\displaystyle 0<|z| the ${\displaystyle \wp }$-function has the following Laurent expansion

${\displaystyle \wp (z)={\frac {1}{z^{2}}}+\sum _{n=1}^{\infty }(2n+1)G_{2n+2}z^{2n}}$

where

${\displaystyle G_{n}=\sum _{0\neq \lambda \in \Lambda }\lambda ^{-n}}$ for ${\displaystyle n\geq 3}$ are so called Eisenstein series.[6]

## Differential equation

Set ${\displaystyle g_{2}=60G_{4}}$ and ${\displaystyle g_{3}=140G_{6}}$. Then the ${\displaystyle \wp }$-function satisfies the differential equation[6]

${\displaystyle \wp '^{2}(z)=4\wp ^{3}(z)-g_{2}\wp (z)-g_{3}}$.

This relation can be verified by forming a linear combination of powers of ${\displaystyle \wp }$ and ${\displaystyle \wp '}$ to eliminate the pole at ${\displaystyle z=0}$. This yields an entire elliptic function that has to be constant by Liouville's theorem .[6]

## Invariants

The real part of the invariant g3 as a function of the nome q on the unit disk.
The imaginary part of the invariant g3 as a function of the nome q on the unit disk.

The coefficients of the above differential equation g2 and g3 are known as the invariants. Because they depend on the lattice ${\displaystyle \Lambda }$ they can be viewed as functions in ${\displaystyle \omega _{1}}$and ${\displaystyle \omega _{2}}$.

The series expansion suggests that g2 and g3 are homogeneous functions of degree −4 and −6. That is[7]

${\displaystyle g_{2}(\lambda \omega _{1},\lambda \omega _{2})=\lambda ^{-4}g_{2}(\omega _{1},\omega _{2})}$
${\displaystyle g_{3}(\lambda \omega _{1},\lambda \omega _{2})=\lambda ^{-6}g_{3}(\omega _{1},\omega _{2})}$ for ${\displaystyle \lambda \neq 0}$.

If ${\displaystyle \omega _{1}}$and ${\displaystyle \omega _{2}}$ are chosen in such a way that ${\displaystyle \operatorname {Im} \left({\frac {\omega _{2}}{\omega _{1}}}\right)>0}$ g2 and g3 can be interpreted as functions on the upper half-plane ${\displaystyle \mathbb {H} :=\{z\in \mathbb {C} :\operatorname {Im} (z)>0\}}$.

Let ${\displaystyle \tau ={\frac {\omega _{2}}{\omega _{1}}}}$. One has:[8]

${\displaystyle g_{2}(1,\tau )=\omega _{1}^{4}g_{2}(\omega _{1},\omega _{2})}$,
${\displaystyle g_{3}(1,\tau )=\omega _{1}^{6}g_{3}(\omega _{1},\omega _{2})}$.

That means g2 and g3 are only scaled by doing this. Set

${\displaystyle g_{2}(\tau ):=g_{2}(1,\tau )}$, ${\displaystyle g_{3}(\tau ):=g_{3}(1,\tau )}$.

As functions of ${\displaystyle \tau \in \mathbb {H} }$ ${\displaystyle g_{2},g_{3}}$ are so called modular forms.

The Fourier series for ${\displaystyle g_{2}}$ and ${\displaystyle g_{3}}$ are given as follows:[9]

${\displaystyle g_{2}(\tau )={\frac {4}{3}}\pi ^{4}\left[1+240\sum _{k=1}^{\infty }\sigma _{3}(k)q^{2k}\right]}$
${\displaystyle g_{3}(\tau )={\frac {8}{27}}\pi ^{6}\left[1-504\sum _{k=1}^{\infty }\sigma _{5}(k)q^{2k}\right]}$

where ${\displaystyle \sigma _{a}(k):=\sum _{d\mid {k}}d^{\alpha }}$ is the divisor function and ${\displaystyle q:=\exp(i\pi \tau )}$.

## Modular discriminant

The real part of the discriminant as a function of the nome q on the unit disk.

The modular discriminant Δ is defined as the discriminant of the polynomial at right-hand side of the above differential equation:

${\displaystyle \Delta =g_{2}^{3}-27g_{3}^{2}.\,}$

The discriminant is a modular form of weight 12. That is, under the action of the modular group, it transforms as

${\displaystyle \Delta \left({\frac {a\tau +b}{c\tau +d}}\right)=\left(c\tau +d\right)^{12}\Delta (\tau )}$

where ${\displaystyle a,b,d,c\in \mathbb {Z} }$ with ad − bc = 1.[10]

Note that ${\displaystyle \Delta =(2\pi )^{12}\eta ^{24}}$ where ${\displaystyle \eta }$ is the Dedekind eta function.[11]

For the Fourier coefficients of ${\displaystyle \Delta }$, see Ramanujan tau function.

## The constants e1, e2 and e3

${\displaystyle e_{1}}$, ${\displaystyle e_{2}}$ and ${\displaystyle e_{3}}$ are usually used to denote the values of the ${\displaystyle \wp }$-function at the half-periods.

${\displaystyle e_{1}\equiv \wp \left({\frac {\omega _{1}}{2}}\right)}$
${\displaystyle e_{2}\equiv \wp \left({\frac {\omega _{2}}{2}}\right)}$
${\displaystyle e_{3}\equiv \wp \left({\frac {\omega _{1}+\omega _{2}}{2}}\right)}$

They are pairwise distinct and only depend on the lattice ${\displaystyle \Lambda }$ and not on its generators.[12]

${\displaystyle e_{1}}$, ${\displaystyle e_{2}}$ and ${\displaystyle e_{3}}$ are the roots of the cubic polynomial ${\displaystyle 4\wp (z)^{3}-g_{2}\wp (z)-g_{3}}$ and are related by the equation:

${\displaystyle e_{1}+e_{2}+e_{3}=0}$.

Because those roots are distinct the discriminant ${\displaystyle \Delta }$ does not vanish on the upper half plane.[13] Now we can rewrite the differential equation:

${\displaystyle \wp '^{2}(z)=4(\wp (z)-e_{1})(\wp (z)-e_{2})(\wp (z)-e_{3})}$.

That means the half-periods are zeros of ${\displaystyle \wp '}$.

The invariants ${\displaystyle g_{2}}$ and ${\displaystyle g_{3}}$ can be expressed in terms of these constants in the following way:[14]

${\displaystyle g_{2}=-4(e_{1}e_{2}+e_{1}e_{3}+e_{2}e_{3})}$
${\displaystyle g_{3}=4e_{1}e_{2}e_{3}}$

## Relation to elliptic curves

Consider the projective cubic curve

${\displaystyle {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} }=\{(x,y)\in \mathbb {C} ^{2}:y^{2}=4x^{3}-g_{2}x+g_{3}\}\cup \{\infty \}\subset \mathbb {P} _{\mathbb {C} }^{2}}$.

For this cubic, also called Weierstrass cubic, there exists no rational parameterization, if ${\displaystyle \Delta \neq 0}$.[2] In this case it is also called an elliptic curve. Nevertheless there is a parameterization that uses the ${\displaystyle \wp }$-function and its derivative ${\displaystyle \wp '}$:[15]

${\displaystyle \varphi :\mathbb {C} /\Lambda \to {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} },\quad {\bar {z}}\mapsto {\begin{cases}(\wp (z),\wp '(z),1)&{\bar {z}}\neq 0\\\infty \quad &{\bar {z}}=0\end{cases}}}$

Now the map ${\displaystyle \varphi }$ is bijective and parameterizes the elliptic curve ${\displaystyle {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} }}$.

${\displaystyle \mathbb {C} /\Lambda }$ is an abelian group and a topological space, equipped with the quotient topology.

It can be shown that every Weierstrass cubic is given in such a way. That is to say that for every pair ${\displaystyle g_{2},g_{3}\in \mathbb {C} }$ with ${\displaystyle \Delta =g_{2}^{3}-27g_{3}^{2}\neq 0}$ there exists a lattice ${\displaystyle \mathbb {Z} \omega _{1}+\mathbb {Z} \omega _{2}}$, such that

${\displaystyle g_{2}=g_{2}(\omega _{1},\omega _{2})}$ and ${\displaystyle g_{3}=g_{3}(\omega _{1},\omega _{2})}$.[16]

The statement that elliptic curves over ${\displaystyle \mathbb {Q} }$ can be parameterized over ${\displaystyle \mathbb {Q} }$, is known as the modularity theorem. This is an important theorem in number theory. It was part of Andrew Wiles' proof (1995) of Fermat's Last Theorem.

Let ${\displaystyle z,w\in \mathbb {C} }$, so that ${\displaystyle z,w,z+w,z-w\notin \Lambda }$. Then one has:[17]

${\displaystyle \wp (z+w)={\frac {1}{4}}\left[{\frac {\wp '(z)-\wp '(w)}{\wp (z)-\wp (w)}}\right]^{2}-\wp (z)-\wp (w)}$.

As well as the duplication formula:[17]

${\displaystyle \wp (2z)={\frac {1}{4}}\left[{\frac {\wp ''(z)}{\wp '(z)}}\right]^{2}-2\wp (z)}$.

These formulas also have a geometric interpretation, if one looks at the elliptic curve ${\displaystyle {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} }}$ together with the mapping ${\displaystyle {\varphi }:\mathbb {C} /\Lambda \to {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} }}$ as in the previous section.

The group structure of ${\displaystyle (\mathbb {C} /\Lambda ,+)}$ translates to the curve ${\displaystyle {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} }}$and can be geometrically interpreted there:

The sum of three pairwise different points ${\displaystyle a,b,c\in {\bar {C}}_{g_{2},g_{3}}^{\mathbb {C} }}$is zero if and only if they lie on the same line in ${\displaystyle \mathbb {P} _{\mathbb {C} }^{2}}$.[18]

This is equivalent to:

${\displaystyle \det \left({\begin{array}{rrr}1&\wp (u+v)&-\wp '(u+v)\\1&\wp (v)&\wp '(v)\\1&\wp (u)&\wp '(u)\\\end{array}}\right)=0}$,

where ${\displaystyle \wp (u)=a}$, ${\displaystyle \wp (v)=b}$ and ${\displaystyle u,v\notin \Lambda }$.[19]

## Relation to Jacobi's elliptic functions

For numerical work, it is often convenient to calculate the Weierstrass elliptic function in terms of Jacobi's elliptic functions.

The basic relations are:[20]

${\displaystyle \wp (z)=e_{3}+{\frac {e_{1}-e_{3}}{\operatorname {sn} ^{2}w}}=e_{2}+(e_{1}-e_{3}){\frac {\operatorname {dn} ^{2}w}{\operatorname {sn} ^{2}w}}=e_{1}+(e_{1}-e_{3}){\frac {\operatorname {cn} ^{2}w}{\operatorname {sn} ^{2}w}}}$

where ${\displaystyle e_{1},e_{2}}$and ${\displaystyle e_{3}}$ are the three roots described above and where the modulus k of the Jacobi functions equals

${\displaystyle k={\sqrt {\frac {e_{2}-e_{3}}{e_{1}-e_{3}}}}}$

and their argument w equals

${\displaystyle w=z{\sqrt {e_{1}-e_{3}}}.}$

## Typography

The Weierstrass's elliptic function is usually written with a rather special, lower case script letter ℘.[footnote 1]

In computing, the letter ℘ is available as \wp in TeX. In Unicode the code point is U+2118 SCRIPT CAPITAL P (HTML &#8472; · &weierp;, &wp;), with the more correct alias weierstrass elliptic function.[footnote 2] In HTML, it can be escaped as &weierp;.

Character information
Preview
Unicode name SCRIPT CAPITAL P / WEIERSTRASS ELLIPTIC FUNCTION
Encodings decimal hex
Unicode 8472 U+2118
UTF-8 226 132 152 E2 84 98
Numeric character reference &#8472; &#x2118;
Named character reference &weierp;, &wp;

## Footnotes

1. ^ This symbol was used already at least in 1890. The first edition of A Course of Modern Analysis by E. T. Whittaker in 1902 also used it.[21]
2. ^ The Unicode Consortium has acknowledged two problems with the letter's name: the letter is in fact lowercase, and it is not a "script" class letter, like U+1D4C5 𝓅 MATHEMATICAL SCRIPT SMALL P, but the letter for Weierstrass's elliptic function. Unicode added the alias as a correction.[22][23]

## References

1. ^ Apostol, Tom M. (1976). Modular functions and Dirichlet series in number theory. New York: Springer-Verlag. p. 9. ISBN 0-387-90185-X. OCLC 2121639.
2. ^ a b Hulek, Klaus. (2012), Elementare Algebraische Geometrie : Grundlegende Begriffe und Techniken mit zahlreichen Beispielen und Anwendungen (in German) (2., überarb. u. erw. Aufl. 2012 ed.), Wiesbaden: Vieweg+Teubner Verlag, p. 8, ISBN 978-3-8348-2348-9
3. ^ Rolf Busam (2006), Funktionentheorie 1 (in German) (4., korr. und erw. Aufl ed.), Berlin: Springer, p. 259, ISBN 978-3-540-32058-6
4. ^ Jeremy Gray (2015), Real and the complex : a history of analysis in the 19th century (in German), Cham, p. 71, ISBN 978-3-319-23715-2
5. ^ Rolf Busam (2006), Funktionentheorie 1 (in German) (4., korr. und erw. Aufl ed.), Berlin: Springer, p. 294, ISBN 978-3-540-32058-6
6. Apostol, Tom M. (1976), Modular functions and Dirichlet series in number theory (in German), New York: Springer-Verlag, p. 11, ISBN 0-387-90185-X
7. ^ Apostol, Tom M. (1976). Modular functions and Dirichlet series in number theory. New York: Springer-Verlag. p. 14. ISBN 0-387-90185-X. OCLC 2121639.
8. ^ Apostol, Tom M. (1976), Modular functions and Dirichlet series in number theory (in German), New York: Springer-Verlag, p. 14, ISBN 0-387-90185-X
9. ^ Apostol, Tom M. (1990). Modular functions and Dirichlet series in number theory (2nd ed.). New York: Springer-Verlag. p. 20. ISBN 0-387-97127-0. OCLC 20262861.
10. ^ Apostol, Tom M. (1976). Modular functions and Dirichlet series in number theory. New York: Springer-Verlag. p. 50. ISBN 0-387-90185-X. OCLC 2121639.
11. ^ Chandrasekharan, K. (Komaravolu), 1920- (1985). Elliptic functions. Berlin: Springer-Verlag. p. 122. ISBN 0-387-15295-4. OCLC 12053023.CS1 maint: multiple names: authors list (link)
12. ^ Rolf Busam (2006), Funktionentheorie 1 (in German) (4., korr. und erw. Aufl ed.), Berlin: Springer, p. 270, ISBN 978-3-540-32058-6
13. ^ Tom M. Apostol (1976), Modular functions and Dirichlet series in number theory (in German), New York: Springer-Verlag, p. 13, ISBN 0-387-90185-X
14. ^ K. Chandrasekharan (1985), Elliptic functions (in German), Berlin: Springer-Verlag, p. 33, ISBN 0-387-15295-4
15. ^ Hulek, Klaus. (2012), Elementare Algebraische Geometrie : Grundlegende Begriffe und Techniken mit zahlreichen Beispielen und Anwendungen (in German) (2., überarb. u. erw. Aufl. 2012 ed.), Wiesbaden: Vieweg+Teubner Verlag, p. 12, ISBN 978-3-8348-2348-9
16. ^ Hulek, Klaus. (2012), Elementare Algebraische Geometrie : Grundlegende Begriffe und Techniken mit zahlreichen Beispielen und Anwendungen (in German) (2., überarb. u. erw. Aufl. 2012 ed.), Wiesbaden: Vieweg+Teubner Verlag, p. 111, ISBN 978-3-8348-2348-9
17. ^ a b Rolf Busam (2006), Funktionentheorie 1 (in German) (4., korr. und erw. Aufl ed.), Berlin: Springer, p. 286, ISBN 978-3-540-32058-6
18. ^ Rolf Busam (2006), Funktionentheorie 1 (in German) (4., korr. und erw. Aufl ed.), Berlin: Springer, p. 287, ISBN 978-3-540-32058-6
19. ^ Rolf Busam (2006), Funktionentheorie 1 (in German) (4., korr. und erw. Aufl ed.), Berlin: Springer, p. 288, ISBN 978-3-540-32058-6
20. ^ Korn GA, Korn TM (1961). Mathematical Handbook for Scientists and Engineers. New York: McGraw–Hill. p. 721. LCCN 59014456.
21. ^ teika kazura (2017-08-17), The letter ℘ Name & origin?, MathOverflow, retrieved 2018-08-30
22. ^ "Known Anomalies in Unicode Character Names". Unicode Technical Note #27. version 4. Unicode, Inc. 2017-04-10. Retrieved 2017-07-20.
23. ^ "NameAliases-10.0.0.txt". Unicode, Inc. 2017-05-06. Retrieved 2017-07-20.