5.2 Elliptic functions and Weierstrass ℘-function

Elliptic functions are complex-valued functions with two independent periods. They're crucial in studying elliptic curves, which have applications in cryptography and number theory. The Weierstrass ℘-function is a key example, providing a link between complex analysis and algebraic geometry.

The ℘-function has unique properties that make it fundamental in elliptic curve theory. It satisfies a specific differential equation, has a Laurent series expansion, and exhibits homogeneity. These characteristics allow us to connect abstract mathematical concepts with practical applications in various fields.

Definition of elliptic functions

• Elliptic functions are a crucial class of functions in complex analysis and algebraic geometry
• They are closely related to elliptic curves and have numerous applications in various fields of mathematics and physics
• Understanding the properties and behavior of elliptic functions is essential for studying elliptic curves and their applications

Doubly periodic functions

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• Elliptic functions are doubly periodic, meaning they are periodic in two independent directions in the complex plane
• For an elliptic function $f(z)$ and two complex numbers $\omega_1$ and $\omega_2$ with $\omega_1/\omega_2 \notin \mathbb{R}$, $f(z+m\omega_1+n\omega_2) = f(z)$ for all $m,n \in \mathbb{Z}$
• The periods $\omega_1$ and $\omega_2$ form a lattice in the complex plane

Poles and residues

• Elliptic functions have a finite number of poles in each period parallelogram
• The sum of the residues at the poles in a period parallelogram is always zero
• This property distinguishes elliptic functions from other meromorphic functions

Liouville's theorem

• Liouville's theorem states that a doubly periodic function that is holomorphic (analytic and single-valued) on the entire complex plane must be constant
• This theorem implies that non-constant elliptic functions must have poles

Fundamental parallelogram

• A fundamental parallelogram is a region in the complex plane that represents the periodicity of an elliptic function
• It is formed by the vectors $\omega_1$ and $\omega_2$, which are the periods of the elliptic function
• The function's behavior within the fundamental parallelogram determines its behavior throughout the entire complex plane

Weierstrass ℘-function

• The Weierstrass $\wp$-function is a specific elliptic function that plays a central role in the theory of elliptic curves
• It is named after the German mathematician Karl Weierstrass, who introduced and studied this function extensively
• The $\wp$-function has several important properties that make it a fundamental object in the study of elliptic functions and elliptic curves

Definition and properties

• The Weierstrass $\wp$-function is defined as a sum over a lattice $\Lambda$ in the complex plane: $\wp(z; \Lambda) = \frac{1}{z^2} + \sum_{\omega \in \Lambda \setminus \{0\}} \left(\frac{1}{(z-\omega)^2} - \frac{1}{\omega^2}\right)$
• It is an even function, meaning $\wp(-z) = \wp(z)$, and has double poles at each lattice point
• The $\wp$-function is doubly periodic with respect to the lattice $\Lambda$

Differential equation

• The Weierstrass $\wp$-function satisfies the following differential equation: $(\wp'(z))^2 = 4\wp(z)^3 - g_2\wp(z) - g_3$
• The constants $g_2$ and $g_3$ are called the invariants of the lattice $\Lambda$ and determine the specific $\wp$-function
• This differential equation is crucial for understanding the connection between $\wp$-functions and elliptic curves

Laurent series expansion

• The Weierstrass $\wp$-function has a Laurent series expansion around $z=0$: $\wp(z) = \frac{1}{z^2} + \sum_{k=1}^\infty (2k+1)G_{2k+2}z^{2k}$
• The coefficients $G_{2k+2}$ are called Eisenstein series and are related to the invariants $g_2$ and $g_3$
• The Laurent series expansion is useful for studying the behavior of the $\wp$-function near its poles

Homogeneity and invariance

• The Weierstrass $\wp$-function is homogeneous of degree $-2$, meaning $\wp(\lambda z; \lambda \Lambda) = \lambda^{-2}\wp(z; \Lambda)$ for any non-zero complex number $\lambda$
• It is also invariant under unimodular transformations of the lattice $\Lambda$, i.e., transformations that preserve the area of the fundamental parallelogram

Poles and residues of ℘-function

• The Weierstrass $\wp$-function has double poles at each lattice point, with a residue of zero
• The behavior of the $\wp$-function near its poles is essential for understanding its properties and connection to elliptic curves
• The Laurent series expansion of the $\wp$-function around a pole provides insight into its local behavior

Algebraic properties of ℘-function

• The Weierstrass $\wp$-function possesses several algebraic properties that are fundamental to its application in the theory of elliptic curves
• These properties include the addition theorem, duplication formula, and division polynomials
• Understanding these algebraic properties is crucial for studying the group structure of elliptic curves and their isogenies

Addition theorem

• The addition theorem for the Weierstrass $\wp$-function states that for any two complex numbers $z_1$ and $z_2$ (excluding poles), $\wp(z_1+z_2) = -\wp(z_1) - \wp(z_2) + \frac{1}{4}\left(\frac{\wp'(z_1)-\wp'(z_2)}{\wp(z_1)-\wp(z_2)}\right)^2$
• This theorem allows the addition of points on an elliptic curve to be expressed in terms of the $\wp$-function and its derivative
• The addition theorem is a key ingredient in the group law of elliptic curves

Duplication formula

• The duplication formula for the Weierstrass $\wp$-function is a special case of the addition theorem when $z_1 = z_2$: $\wp(2z) = -2\wp(z) + \frac{1}{4}\left(\frac{\wp''(z)}{\wp'(z)}\right)^2$
• This formula is useful for studying the doubling of points on an elliptic curve and has applications in elliptic curve cryptography

Division polynomials

• Division polynomials are a sequence of polynomials $\psi_n(x,y)$ that are related to the division of points on an elliptic curve
• They can be defined using the Weierstrass $\wp$-function and its derivatives
• Division polynomials play a crucial role in the study of torsion points and isogenies of elliptic curves

Relation to elliptic curves

• The Weierstrass $\wp$-function and its derivative $\wp'$ provide a parameterization of elliptic curves
• An elliptic curve in Weierstrass form can be written as $y^2 = 4x^3 - g_2x - g_3$, where $g_2$ and $g_3$ are the invariants of the associated lattice
• The map $(z, \Lambda) \mapsto (\wp(z; \Lambda), \wp'(z; \Lambda))$ establishes an isomorphism between the complex torus $\mathbb{C}/\Lambda$ and the elliptic curve

Eisenstein series and ℘-function

• Eisenstein series are a family of modular forms that are closely related to the Weierstrass $\wp$-function
• They play a crucial role in the study of elliptic functions and modular forms
• Understanding the connection between Eisenstein series and the $\wp$-function provides insight into the modularity properties of elliptic curves

Definition of Eisenstein series

• The Eisenstein series $G_{2k}(\Lambda)$ of weight $2k$ for a lattice $\Lambda$ is defined as $G_{2k}(\Lambda) = \sum_{\omega \in \Lambda \setminus \{0\}} \omega^{-2k}$
• These series converge absolutely for $k \geq 2$ and are modular forms of weight $2k$ for the modular group $\text{SL}_2(\mathbb{Z})$
• The Eisenstein series $G_4$ and $G_6$ are particularly important, as they are related to the invariants $g_2$ and $g_3$ of the Weierstrass $\wp$-function

Connection to ℘-function

• The invariants $g_2$ and $g_3$ of the Weierstrass $\wp$-function can be expressed in terms of the Eisenstein series: $g_2 = 60G_4$ and $g_3 = 140G_6$
• The Laurent series expansion of the $\wp$-function involves the Eisenstein series as coefficients: $\wp(z) = \frac{1}{z^2} + \sum_{k=1}^\infty (2k+1)G_{2k+2}z^{2k}$
• This connection highlights the modular properties of the $\wp$-function and its relation to the theory of modular forms

Modularity of Eisenstein series

• The Eisenstein series $G_{2k}$ are modular forms of weight $2k$ for the modular group $\text{SL}_2(\mathbb{Z})$
• This means that they satisfy certain transformation properties under the action of $\text{SL}_2(\mathbb{Z})$ on the upper half-plane
• The modularity of Eisenstein series is a key ingredient in the proof of the modularity theorem for elliptic curves over $\mathbb{Q}$

Lattices and ℘-function

• The Weierstrass $\wp$-function is closely related to lattices in the complex plane
• The properties of the $\wp$-function, such as its periods and invariants, are determined by the associated lattice
• Understanding the relationship between lattices and the $\wp$-function is essential for studying the arithmetic and geometric properties of elliptic curves

Lattice invariants g₂ and g₃

• The invariants $g_2$ and $g_3$ of a lattice $\Lambda$ are defined as $g_2(\Lambda) = 60\sum_{\omega \in \Lambda \setminus \{0\}} \omega^{-4}$ and $g_3(\Lambda) = 140\sum_{\omega \in \Lambda \setminus \{0\}} \omega^{-6}$
• These invariants determine the Weierstrass $\wp$-function associated with the lattice $\Lambda$ up to a constant factor
• The discriminant of the lattice is given by $\Delta = g_2^3 - 27g_3^2$ and is non-zero for non-degenerate lattices

Relation between lattices and ℘-function

• The Weierstrass $\wp$-function $\wp(z; \Lambda)$ is uniquely determined by its associated lattice $\Lambda$
• The periods of the $\wp$-function are given by the basis vectors of the lattice
• The invariants $g_2$ and $g_3$ of the $\wp$-function are determined by the lattice invariants $g_2(\Lambda)$ and $g_3(\Lambda)$

Isomorphic lattices and ℘-functions

• Two lattices $\Lambda_1$ and $\Lambda_2$ are isomorphic if there exists a non-zero complex number $\lambda$ such that $\Lambda_2 = \lambda \Lambda_1$
• Isomorphic lattices give rise to homothetic $\wp$-functions, i.e., $\wp(z; \Lambda_2) = \lambda^{-2}\wp(\lambda^{-1}z; \Lambda_1)$
• The $j$-invariant, defined as $j(\Lambda) = 1728\frac{g_2^3}{\Delta}$, is an important quantity that characterizes isomorphism classes of lattices and elliptic curves

Complex tori and ℘-function

• Complex tori, which are quotients of the complex plane by a lattice, are closely related to elliptic curves and the Weierstrass $\wp$-function
• The $\wp$-function provides a natural parameterization of complex tori and establishes an isomorphism between complex tori and elliptic curves
• Studying complex tori and their relationship to the $\wp$-function is crucial for understanding the geometry and arithmetic of elliptic curves

Uniformization of complex tori

• Every complex torus $\mathbb{C}/\Lambda$ can be uniformized by the complex plane $\mathbb{C}$ using the Weierstrass $\wp$-function
• The map $z \mapsto (\wp(z; \Lambda), \wp'(z; \Lambda))$ defines an isomorphism between the complex torus $\mathbb{C}/\Lambda$ and the elliptic curve $y^2 = 4x^3 - g_2(\Lambda)x - g_3(\Lambda)$
• This uniformization provides a powerful tool for studying the geometry and topology of complex tori and elliptic curves

Elliptic curves as complex tori

• Every elliptic curve over $\mathbb{C}$ can be realized as a complex torus $\mathbb{C}/\Lambda$ for some lattice $\Lambda$
• The Weierstrass $\wp$-function associated with the lattice $\Lambda$ provides a parameterization of the elliptic curve
• This perspective allows the application of complex analytic techniques to the study of elliptic curves

Isogenies and complex multiplication

• An isogeny between two elliptic curves is a surjective holomorphic map that preserves the group structure
• Isogenies can be studied using the Weierstrass $\wp$-function and the associated lattices
• Elliptic curves with complex multiplication, i.e., those admitting an isogeny to themselves, have lattices with special arithmetic properties and are of particular interest in number theory

Applications of ℘-function

• The Weierstrass $\wp$-function has numerous applications in various areas of mathematics and physics
• Its connection to elliptic integrals, as well as its appearance in physical models and cryptographic protocols, highlights its importance and versatility
• Exploring the applications of the $\wp$-function provides insight into the practical significance of elliptic functions and elliptic curves

Elliptic integrals and ℘-function

• Elliptic integrals, such as $\int \frac{dx}{\sqrt{4x^3 - g_2x - g_3}}$, can be evaluated using the Weierstrass $\wp$-function
• The inverse of the $\wp$-function, called the Weierstrass elliptic function, is related to elliptic integrals and provides a way to express them in terms of elliptic functions
• This connection is important in the study of various physical problems, such as the motion of a pendulum or the dynamics of a spinning top

Elliptic functions in physics

• Elliptic functions, including the Weierstrass $\wp$-function, appear naturally in several physical models
• They are used to describe the motion of particles in periodic potentials, such as in the study of crystal lattices or the dynamics of electrons in a solid
• Elliptic functions also play a role in the theory of integrable systems, such as the Korteweg-de Vries equation and the sine-Gordon equation

Elliptic functions in cryptography

• Elliptic curves and elliptic functions have found important applications in cryptography
• The group structure of elliptic curves over finite fields is used in the construction of public-key cryptographic protocols, such as the Elliptic Curve Digital Signature Algorithm (ECDSA) and the Elliptic Curve Diffie-Hellman (ECDH) key exchange
• The Weierstrass $\wp$-function and its properties are essential for understanding the arithmetic of elliptic curves and their use in cryptographic applications