In
mathematics,
Felix Klein
Christian Felix Klein (; 25 April 1849 – 22 June 1925) was a German mathematician and mathematics educator, known for his work with group theory, complex analysis, non-Euclidean geometry, and on the associations between geometry and grou ...
's -invariant or function, regarded as a function of a
complex variable , is a
modular function
In mathematics, a modular form is a (complex) analytic function on the upper half-plane satisfying a certain kind of functional equation with respect to the group action of the modular group, and also satisfying a growth condition. The theory of ...
of weight zero for defined on the
upper half-plane
In mathematics, the upper half-plane, \,\mathcal\,, is the set of points in the Cartesian plane with > 0.
Complex plane
Mathematicians sometimes identify the Cartesian plane with the complex plane, and then the upper half-plane corresponds to ...
of
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the fo ...
s. It is the unique such function which is
holomorphic
In mathematics, a holomorphic function is a complex-valued function of one or more complex variables that is complex differentiable in a neighbourhood of each point in a domain in complex coordinate space . The existence of a complex derivati ...
away from a simple pole at the
cusp
A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth.
Cusp or CUSP may also refer to:
Mathematics
* Cusp (singularity), a singular point of a curve
* Cusp catastrophe, a branch of bifurc ...
such that
:
Rational functions of are modular, and in fact give all modular functions. Classically, the -invariant was studied as a parameterization of
elliptic curve
In mathematics, an elliptic curve is a smooth, projective, algebraic curve of genus one, on which there is a specified point . An elliptic curve is defined over a field and describes points in , the Cartesian product of with itself. If ...
s over , but it also has surprising connections to the symmetries of the
Monster group
In the area of abstract algebra known as group theory, the monster group M (also known as the Fischer–Griess monster, or the friendly giant) is the largest sporadic simple group, having order
24632059761121331719232931414759 ...
(this connection is referred to as
monstrous moonshine
In mathematics, monstrous moonshine, or moonshine theory, is the unexpected connection between the monster group ''M'' and modular functions, in particular, the ''j'' function. The term was coined by John Conway and Simon P. Norton in 1979. ...
).
Definition
The -invariant can be defined as a function on the
upper half-plane
In mathematics, the upper half-plane, \,\mathcal\,, is the set of points in the Cartesian plane with > 0.
Complex plane
Mathematicians sometimes identify the Cartesian plane with the complex plane, and then the upper half-plane corresponds to ...
:
with the third definition implying
can be expressed as a
cube, also since
1728 =
.
The given functions are the
modular discriminant
In mathematics, the Weierstrass 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 ℘-functions and they are usually denoted by the ...
,
Dedekind eta function
In mathematics, the Dedekind eta function, named after Richard Dedekind, is a modular form of weight 1/2 and is a function defined on the upper half-plane of complex numbers, where the imaginary part is positive. It also occurs in bosonic string ...
, and modular invariants,
:
:
where
,
are
Fourier series,
:
and
,
are
Eisenstein series
Eisenstein series, named after German mathematician Gotthold Eisenstein, are particular modular forms with infinite series expansions that may be written down directly. Originally defined for the modular group, Eisenstein series can be generaliz ...
,
:
and
(the square of the
nome). The -invariant can then be directly expressed in terms of the Eisenstein series as,
:
with no numerical factor other than 1728. This implies a third way to define the modular discriminant,
:
For example, using the definitions above and
, then the Dedekind eta function
has the
exact value,
:
implying the
transcendental numbers
In mathematics, a transcendental number is a number that is not algebraic—that is, not the root of a non-zero polynomial of finite degree with rational coefficients. The best known transcendental numbers are and .
Though only a few classes o ...
,
:
but yielding the
algebraic number (in fact, an
integer
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
),
:
In general, this can be motivated by viewing each as representing an isomorphism class of elliptic curves. Every elliptic curve over is a complex torus, and thus can be identified with a rank 2 lattice; that is, a two-dimensional lattice of . This lattice can be rotated and scaled (operations that preserve the isomorphism class), so that it is generated by and . This lattice corresponds to the elliptic curve
(see
Weierstrass elliptic functions
In mathematics, the Weierstrass 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 ℘-functions and they are usually denoted by the ...
).
Note that is defined everywhere in as the modular discriminant is non-zero. This is due to the corresponding cubic polynomial having distinct roots.
The fundamental region
It can be shown that is a
modular form of weight twelve, and one of weight four, so that its third power is also of weight twelve. Thus their quotient, and therefore , is a modular function of weight zero, in particular a holomorphic function invariant under the action of . Quotienting out by its centre yields the
modular group, which we may identify with the
projective special linear group .
By a suitable choice of transformation belonging to this group,
:
we may reduce to a value giving the same value for , and lying in the
fundamental region
Given a topological space and a group acting on it, the images of a single point under the group action form an orbit of the action. A fundamental domain or fundamental region is a subset of the space which contains exactly one point from each o ...
for , which consists of values for satisfying the conditions
:
The function when restricted to this region still takes on every value in the
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the fo ...
s exactly once. In other words, for every in , there is a unique τ in the fundamental region such that . Thus, has the property of mapping the fundamental region to the entire complex plane.
Additionally two values produce the same elliptic curve iff for some . This means provides a bijection from the set of elliptic curves over to the complex plane.
As a Riemann surface, the fundamental region has genus , and every (
level one) modular function is a
rational function in ; and, conversely, every rational function in is a modular function. In other words, the field of modular functions is .
Class field theory and
The -invariant has many remarkable properties:
*If is any
CM point, that is, any element of an imaginary
quadratic field
In algebraic number theory, a quadratic field is an algebraic number field of degree two over \mathbf, the rational numbers.
Every such quadratic field is some \mathbf(\sqrt) where d is a (uniquely defined) square-free integer different from 0 a ...
with positive imaginary part (so that is defined), then is an
algebraic integer
In algebraic number theory, an algebraic integer is a complex number which is integral over the integers. That is, an algebraic integer is a complex root of some monic polynomial (a polynomial whose leading coefficient is 1) whose coefficients ...
. These special values are called
singular moduli.
* The field extension is abelian, that is, it has an abelian
Galois group
In mathematics, in the area of abstract algebra known as Galois theory, the Galois group of a certain type of field extension is a specific group associated with the field extension. The study of field extensions and their relationship to the po ...
.
* Let be the lattice in generated by It is easy to see that all of the elements of which fix under multiplication form a ring with units, called an
order. The other lattices with generators associated in like manner to the same order define the
algebraic conjugates of over . Ordered by inclusion, the unique maximal order in is the ring of algebraic integers of , and values of having it as its associated order lead to
unramified extension
In geometry, ramification is 'branching out', in the way that the square root function, for complex numbers, can be seen to have two ''branches'' differing in sign. The term is also used from the opposite perspective (branches coming together) as ...
s of .
These classical results are the starting point for the theory of
complex multiplication
In mathematics, complex multiplication (CM) is the theory of elliptic curves ''E'' that have an endomorphism ring larger than the integers. Put another way, it contains the theory of elliptic functions with extra symmetries, such as are visible wh ...
.
Transcendence properties
In 1937
Theodor Schneider __NOTOC__
Theodor Schneider (7 May 1911, Frankfurt am Main – 31 October 1988, Freiburg im Breisgau) was a German mathematician, best known for providing proof of what is now known as the Gelfond–Schneider theorem.
Schneider studied from 19 ...
proved the aforementioned result that if is a quadratic irrational number in the upper half plane then is an algebraic integer. In addition he proved that if is an
algebraic number but not imaginary quadratic then is transcendental.
The function has numerous other transcendental properties.
Kurt Mahler
Kurt Mahler FRS (26 July 1903, Krefeld, Germany – 25 February 1988, Canberra, Australia) was a German mathematician who worked in the fields of transcendental number theory, diophantine approximation, ''p''-adic analysis, and the geometry of ...
conjectured a particular transcendence result that is often referred to as Mahler's conjecture, though it was proved as a corollary of results by Yu. V. Nesterenko and Patrice Phillipon in the 1990s. Mahler's conjecture was that if was in the upper half plane then and were never both simultaneously algebraic. Stronger results are now known, for example if is algebraic then the following three numbers are algebraically independent, and thus at least two of them transcendental:
:
The -expansion and moonshine
Several remarkable properties of have to do with its
-expansion (
Fourier series expansion), written as a
Laurent series in terms of , which begins:
:
Note that has a
simple pole at the cusp, so its -expansion has no terms below .
All the Fourier coefficients are integers, which results in several
almost integers, notably
Ramanujan's constant:
:
.
The
asymptotic formula
In mathematical analysis, asymptotic analysis, also known as asymptotics, is a method of describing limiting behavior.
As an illustration, suppose that we are interested in the properties of a function as becomes very large. If , then as bec ...
for the coefficient of is given by
:
,
as can be proved by the
Hardy–Littlewood circle method
In mathematics, the Hardy–Littlewood circle method is a technique of analytic number theory. It is named for G. H. Hardy and J. E. Littlewood, who developed it in a series of papers on Waring's problem.
History
The initial idea is usually at ...
.
Moonshine
More remarkably, the Fourier coefficients for the positive exponents of are the dimensions of the graded part of an infinite-dimensional
graded algebra representation of the
monster group
In the area of abstract algebra known as group theory, the monster group M (also known as the Fischer–Griess monster, or the friendly giant) is the largest sporadic simple group, having order
24632059761121331719232931414759 ...
called the ''
moonshine module
The monster vertex algebra (or moonshine module) is a vertex algebra acted on by the monster group that was constructed by Igor Frenkel, James Lepowsky, and Arne Meurman. R. Borcherds used it to prove the monstrous moonshine conjectures, by apply ...
'' – specifically, the coefficient of is the dimension of grade- part of the moonshine module, the first example being the
Griess algebra In mathematics, the Griess algebra is a commutative non-associative algebra on a real vector space of dimension 196884 that has the Monster group ''M'' as its automorphism group. It is named after mathematician R. L. Griess, who constructed it in ...
, which has dimension 196,884, corresponding to the term . This startling observation, first made by
John McKay, was the starting point for
moonshine theory
In mathematics, monstrous moonshine, or moonshine theory, is the unexpected connection between the monster group ''M'' and modular functions, in particular, the ''j'' function. The term was coined by John Conway and Simon P. Norton in 197 ...
.
The study of the Moonshine conjecture led
John Horton Conway
John Horton Conway (26 December 1937 – 11 April 2020) was an English mathematician active in the theory of finite groups, knot theory, number theory, combinatorial game theory and coding theory. He also made contributions to many branches ...
and
Simon P. Norton to look at the genus-zero modular functions. If they are normalized to have the form
:
then
John G. Thompson
John Griggs Thompson (born October 13, 1932) is an American mathematician at the University of Florida noted for his work in the field of finite groups. He was awarded the Fields Medal in 1970, the Wolf Prize in 1992, and the Abel Prize in 2008.
...
showed that there are only a finite number of such functions (of some finite level), and Chris J. Cummins later showed that there are exactly 6486 of them, 616 of which have integral coefficients.
Alternate expressions
We have
:
where and is the
modular lambda function
:
a ratio of
Jacobi theta functions , and is the square of the elliptic modulus .
[Chandrasekharan (1985) p.108] The value of is unchanged when is replaced by any of the six values of the
cross-ratio
In geometry, the cross-ratio, also called the double ratio and anharmonic ratio, is a number associated with a list of four collinear points, particularly points on a projective line. Given four points ''A'', ''B'', ''C'' and ''D'' on a line, th ...
:
:
The branch points of are at , so that is a
Belyi function
In mathematics, Belyi's theorem on algebraic curves states that any non-singular algebraic curve ''C'', defined by algebraic number coefficients, represents a compact Riemann surface which is a ramified covering of the Riemann sphere, ramified at ...
.
Expressions in terms of theta functions
Define the
nome and the
Jacobi theta function
In mathematics, theta functions are special functions of several complex variables. They show up in many topics, including Abelian varieties, moduli spaces, quadratic forms, and solitons. As Grassmann algebras, they appear in quantum field theo ...
,
:
from which one can derive the
auxiliary theta functions. Let,
:
where and are alternative notations, and . Then we have the for
modular invariants , ,
:
and modular discriminant,
:
with
Dedekind eta function
In mathematics, the Dedekind eta function, named after Richard Dedekind, is a modular form of weight 1/2 and is a function defined on the upper half-plane of complex numbers, where the imaginary part is positive. It also occurs in bosonic string ...
. The can then be rapidly computed,
:
Algebraic definition
So far we have been considering as a function of a complex variable. However, as an invariant for isomorphism classes of elliptic curves, it can be defined purely algebraically. Let
:
be a plane elliptic curve over any field. Then we may perform successive transformations to get the above equation into the standard form (note that this transformation can only be made when the characteristic of the field is not equal to 2 or 3). The resulting coefficients are:
:
where and . We also have the
discriminant
:
The -invariant for the elliptic curve may now be defined as
:
In the case that the field over which the curve is defined has characteristic different from 2 or 3, this is equal to
:
Inverse function
The
inverse function
In mathematics, the inverse function of a function (also called the inverse of ) is a function that undoes the operation of . The inverse of exists if and only if is bijective, and if it exists, is denoted by f^ .
For a function f\colon X ...
of the -invariant can be expressed in terms of the
hypergeometric function (see also the article
Picard–Fuchs equation). Explicitly, given a number , to solve the equation for can be done in at least four ways.
Method 1: Solving the
sextic in ,
:
where , and is the
modular lambda function so the sextic can be solved as a cubic in . Then,
:
for any of the six values of , where is the
arithmetic–geometric mean.
[The equality holds if the arithmetic–geometric mean of ]complex numbers
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the form ...
(such that ) is defined as follows: Let , , , where the signs are chosen such that for all . If , the sign is chosen such that . Then . When are positive real (with ), this definition coincides with the usual definition of the arithmetic–geometric mean for positive real numbers. Se
The Arithmetic-Geometric Mean of Gauss
by David A. Cox.
Method 2: Solving the
quartic in ,
:
then for any of the four
roots,
:
Method 3: Solving the
cubic in ,
:
then for any of the three roots,
:
Method 4: Solving the
quadratic in ,
:
then,
:
One root gives , and the other gives , but since , it makes no difference which is chosen. The latter three methods can be found in
Ramanujan's theory of
elliptic functions to alternative bases.
The inversion is applied in high-precision calculations of elliptic function periods even as their ratios become unbounded. A related result is the expressibility via quadratic radicals of the values of at the points of the imaginary axis whose magnitudes are powers of 2 (thus permitting
compass and straightedge constructions). The latter result is hardly evident since the
modular equation for of order 2 is cubic.
Pi formulas
The
Chudnovsky brothers
David Volfovich Chudnovsky (born January 22, 1947 in Kyiv) and Gregory Volfovich Chudnovsky (born April 17, 1952 in Kyiv) are Ukrainian-born American mathematicians and engineers known for their world-record mathematical calculations and developing ...
found in 1987,
[.]
:
a proof of which uses the fact that
:
For similar formulas, see the
Ramanujan–Sato series.
Special values
The -invariant vanishes at the "corner" of the
fundamental domain:
:
Here are a few more special values given in terms of the alternative notation , the first four well known:
:
Failure to classify elliptic curves over other fields
The
-invariant is only sensitive to isomorphism classes of elliptic curves over the complex numbers, or more generally, an
algebraically closed field. Over other fields there exist examples of elliptic curves whose
-invariant is the same, but are non-isomorphic. For example, let
be the elliptic curves associated to the polynomials
both having
-invariant
. Then, the rational points of
can be computed as:
since
There are no rational solutions with
. This can be shown using
Cardano's formula
In algebra, a cubic equation in one variable is an equation of the form
:ax^3+bx^2+cx+d=0
in which is nonzero.
The solutions of this equation are called roots of the cubic function defined by the left-hand side of the equation. If all of the ...
to show that in that case the solutions to
are all irrational.
On the other hand, on the set of points
the equation for
becomes
. Dividing by
to eliminate the
solution, the quadratic formula gives the rational solutions:
If these curves are considered over
, there is an isomorphism
sending
References
Notes
Other
*. Provides a very readable introduction and various interesting identities.
**
*. Provides a variety of interesting algebraic identities, including the inverse as a hypergeometric series.
* Introduces the j-invariant and discusses the related class field theory.
*. Includes a list of the 175 genus-zero modular functions.
*. Provides a short review in the context of modular forms.
*{{citation, first=Theodor, last=Schneider, author-link=Theodor Schneider, title=Arithmetische Untersuchungen elliptischer Integrale, journal=Math. Annalen, volume=113, year=1937, pages=1–13, mr=1513075, doi=10.1007/BF01571618, s2cid=121073687.
Modular forms
Elliptic functions
Moonshine theory