# §23.3 Differential Equations

## §23.3(i) Invariants, Roots, and Discriminant

The lattice invariants are defined by

 23.3.1 $\displaystyle g_{2}$ $\displaystyle=60\sum_{w\in\mathbb{L}\setminus\{0\}}w^{-4},$ Symbols: $\in$: element of, $\setminus$: set subtraction, $\mathbb{L}$: lattice and $g_{2}$, $g_{3}$: lattice invariants A&S Ref: 18.1.1 Referenced by: §23.3(i) Permalink: http://dlmf.nist.gov/23.3.E1 Encodings: TeX, pMML, png 23.3.2 $\displaystyle g_{3}$ $\displaystyle=140\sum_{w\in\mathbb{L}\setminus\{0\}}w^{-6}.$ Symbols: $\in$: element of, $\setminus$: set subtraction, $\mathbb{L}$: lattice and $g_{2}$, $g_{3}$: lattice invariants A&S Ref: 18.1.1 Referenced by: §23.3(i) Permalink: http://dlmf.nist.gov/23.3.E2 Encodings: TeX, pMML, png

The lattice roots satisfy the cubic equation

 23.3.3 $4z^{3}-g_{2}z-g_{3}=0,$ Symbols: $g_{2}$, $g_{3}$: lattice invariants and $z$: complex A&S Ref: 18.1.13 Referenced by: §23.3(ii) Permalink: http://dlmf.nist.gov/23.3.E3 Encodings: TeX, pMML, png

and are denoted by $e_{1},e_{2},e_{3}$. The discriminant1.11(ii)) is given by

 23.3.4 $\Delta=g_{2}^{3}-27g_{3}^{2}=16(e_{2}-e_{3})^{2}(e_{3}-e_{1})^{2}(e_{1}-e_{2})% ^{2}.$ Symbols: $g_{2}$, $g_{3}$: lattice invariants, $e_{j}$: zeros and $\Delta$: discriminant A&S Ref: 18.1.8 Referenced by: §23.19 Permalink: http://dlmf.nist.gov/23.3.E4 Encodings: TeX, pMML, png

In consequence,

 23.3.5 $e_{1}+e_{2}+e_{3}=0,$ Symbols: $e_{j}$: zeros A&S Ref: 18.1.11 Permalink: http://dlmf.nist.gov/23.3.E5 Encodings: TeX, pMML, png
 23.3.6 $g_{2}=2(e_{1}^{2}+e_{2}^{2}+e_{3}^{2})=-4(e_{2}e_{3}+e_{3}e_{1}+e_{1}e_{2}),$ Symbols: $g_{2}$, $g_{3}$: lattice invariants and $e_{j}$: zeros A&S Ref: 18.1.9 Referenced by: §23.22(ii) Permalink: http://dlmf.nist.gov/23.3.E6 Encodings: TeX, pMML, png
 23.3.7 $g_{3}=4e_{1}e_{2}e_{3}=\tfrac{4}{3}(e_{1}^{3}+e_{2}^{3}+e_{3}^{3}).$ Symbols: $g_{2}$, $g_{3}$: lattice invariants and $e_{j}$: zeros A&S Ref: 18.1.10 Referenced by: §23.22(ii) Permalink: http://dlmf.nist.gov/23.3.E7 Encodings: TeX, pMML, png

Let $g_{2}^{3}\neq 27g_{3}^{2}$, or equivalently $\Delta$ be nonzero, or $e_{1},e_{2},e_{3}$ be distinct. Given $g_{2}$ and $g_{3}$ there is a unique lattice $\mathbb{L}$ such that (23.3.1) and (23.3.2) are satisfied. We may therefore define

 23.3.8 $\mathop{\wp\/}\nolimits\!\left(z;g_{2},g_{3}\right)=\mathop{\wp\/}\nolimits\!% \left(z|\mathbb{L}\right).$ Defines: $\mathop{\wp\/}\nolimits\!\left(z;g_{2},g_{3}\right)$: Weierstrass $\mathop{\wp\/}\nolimits$-function Symbols: $\mathop{\wp\/}\nolimits\!\left(z\right)$ (= $\mathop{\wp\/}\nolimits\!\left(z|\mathbb{L}\right)$ = $\mathop{\wp\/}\nolimits\!\left(z;g_{2},g_{3}\right)$): Weierstrass $\mathop{\wp\/}\nolimits$-function, $\mathbb{L}$: lattice, $g_{2}$, $g_{3}$: lattice invariants and $z$: complex Permalink: http://dlmf.nist.gov/23.3.E8 Encodings: TeX, pMML, png

Similarly for $\mathop{\zeta\/}\nolimits\!\left(z;g_{2},g_{3}\right)$ and $\mathop{\sigma\/}\nolimits\!\left(z;g_{2},g_{3}\right)$. As functions of $g_{2}$ and $g_{3}$, $\mathop{\wp\/}\nolimits\!\left(z;g_{2},g_{3}\right)$ and $\mathop{\zeta\/}\nolimits\!\left(z;g_{2},g_{3}\right)$ are meromorphic and $\mathop{\sigma\/}\nolimits\!\left(z;g_{2},g_{3}\right)$ is entire.

Conversely, $g_{2}$, $g_{3}$, and the set $\{e_{1},e_{2},e_{3}\}$ are determined uniquely by the lattice $\mathbb{L}$ independently of the choice of generators. However, given any pair of generators $2\omega_{1}$, $2\omega_{3}$ of $\mathbb{L}$, and with $\omega_{2}$ defined by (23.2.1), we can identify the $e_{j}$ individually, via

 23.3.9 $e_{j}=\mathop{\wp\/}\nolimits\!\left(\omega_{j}|\mathbb{L}\right),$ $j=1,2,3$.

In what follows, it will be assumed that (23.3.9) always applies.

## §23.3(ii) Differential Equations and Derivatives

 23.3.10 ${\mathop{\wp\/}\nolimits'}^{2}(z)=4\!{\mathop{\wp\/}\nolimits^{3}}\!\left(z% \right)-g_{2}\mathop{\wp\/}\nolimits\!\left(z\right)-g_{3},$ Symbols: $\mathop{\wp\/}\nolimits\!\left(z;g_{2},g_{3}\right)$: Weierstrass $\mathop{\wp\/}\nolimits$-function, $g_{2}$, $g_{3}$: lattice invariants and $z$: complex A&S Ref: 18.1.6 and 18.6.3 Referenced by: §23.22(ii), §23.3(ii) Permalink: http://dlmf.nist.gov/23.3.E10 Encodings: TeX, pMML, png
 23.3.11 ${\mathop{\wp\/}\nolimits'}^{2}(z)=4(\mathop{\wp\/}\nolimits\!\left(z\right)-e_% {1})(\mathop{\wp\/}\nolimits\!\left(z\right)-e_{2})(\mathop{\wp\/}\nolimits\!% \left(z\right)-e_{3}),$
 23.3.12 $\mathop{\wp\/}\nolimits''\!\left(z\right)=6\!{\mathop{\wp\/}\nolimits^{2}}\!% \left(z\right)-\tfrac{1}{2}g_{2},$
 23.3.13 $\mathop{\wp\/}\nolimits'''\!\left(z\right)=12\mathop{\wp\/}\nolimits\!\left(z% \right)\mathop{\wp\/}\nolimits'\!\left(z\right).$