ellipses and elliptic curves m. ram murty queen’s university
TRANSCRIPT
ELLIPSES ANDELLIPTIC CURVES
M. Ram MurtyQueen’s University
Planetary orbits are elliptical
What is an ellipse?
x2
a2 + y2
b2 = 1
An ellipse has two foci
From: gomath.com/geometry/ellipse.php
Metric mishap causes loss of Mars orbiter (Sept. 30, 1999)
How to calculate arc length
Let f : [0;1] ! R2 be given by t 7! (x(t);y(t)).
By Pythagoras, ¢ s =p
(¢ x)2 + (¢ y)2
ds =
r¡
dxdt
¢2 +³
dydt
´2dt
Arc length =R1
0
r¡
dxdt
¢2 +³
dydt
´2dt
Circumference of an ellipseWecan parametrizethepointsof an ellipsein the¯rst quadrant byf : [0;¼=2]! (asint;bcost):
Circumference =4
R¼=20
pa2 cos2 t + b2 sin2 tdt:
Observe that if a = b, we get 2¼a.
Wecan simplify this as follows.Let cos2 µ= 1¡ sin2 µ.The integral becomes4a
R¼=20
p1¡ ¸ sin2 µdµ
Where¸ = 1¡ b2=a2.
Wecan expand thesquare root usingthebinomial theorem:4a
R¼=20
P 1n=0
³1=2n
´(¡ 1)n¸n sin2n µdµ:
We can use the fact that2
R¼=20
sin2n µdµ= ¼(1=2)(1=2)+1)¢¢¢((1=2)+(n¡ 1))n!
The final answerThe circumference is given by2¼aF (1=2;¡ 1=2;1;¸)whereF (a;b;c;z) =
P 1n=0
(a)n (b)nn!(c)n
zn
is the hypergeometric series and(a)n = a(a+ 1)(a+ 2) ¢¢¢(a+ n ¡ 1):
We now use Landens transformation:F (a;b;2b; 4x
(1+x)2 ) = (1+ x)2aF (a;a ¡ b+ 1=2;b+ 1=2;x2).
The answer can now be written as ¼(a+ b)F (¡ 1=2;¡ 1=2;1;x2).
We put a = ¡ 1=2;b= 1=2 and x = (a¡ b)=(a+ b).
Approximations
• In 1609, Kepler used the approximation (a+b). The above formula shows the perimeter is always greater than this amount.
• In 1773, Euler gave the approximation 2√(a²+b²)/2.
• In 1914, Ramanujan gave the approximation (3(a+b) -√(a+3b)(3a+b)).
What kind of number is this?
For example, if a and bare rational,is the circumeference irrational?
In case a = bwe have a circleand the circumference 2¼ais irrational.
In fact, ¼is transcendental.
What does this mean?
In 1882, Ferdinand von Lindemannproved that ¼is transcendental.
This means that ¼does not satisfyan equation of the typexn +an¡ 1xn¡ 1 +¢¢¢+a1x +a0 = 0with ai rational numbers.
1852-1939
This means that you can’t square the circle!
• There is an ancient problem of constructing a square with straightedge and compass whose area equals .
• Theorem: if you can construct a line segment of length α then α is an algebraic number.
• Since is not algebraic, neither is √.
Some other interesting numbers
• The number e is transcendental.
• This was first proved by Charles Hermite (1822-1901) in 1873.
Is ¼+ e transcendental?
Answer: unknown.Is ¼e transcendental?Answer: unknown.
Not both can be algebraic!
• Here’s a proof.
• If both are algebraic, then(x ¡ e)(x¡ ¼) = x2 ¡ (e+ ¼)x + ¼eis a quadratic polynomial with algebraic coe±cients.This implies both e and ¼are algebraic,a contradiction to the theorems ofHermite and Lindemann.
Conjecture:¼and e are algebraically independent.
But what about the case of the ellipse?
Lets look at the integral again.
Yes.This is a theorem ofTheodor Schneider (1911-1988)
Is the integralR¼=20
pa2 cos2 t + b2 sin2 tdt
transcendentalif a and bare rational?
Putting u = sint the integral becomes
Set k2 = 1¡ b2=a2:
The integral becomes
aR1
0
q1¡ k2u2
1¡ u2 du:
Put t = 1¡ k2u2:
R10
qa2¡ (a2¡ b2)u2
1¡ u2 du:
12
R11¡ k2
tdtpt(t¡ 1)(t¡ (1¡ k2))
Elliptic Integrals
Integrals of the formRdxp
x3+a2x2+a3x+a4are called elliptic integrals of the ¯rst kind.
Integrals of the formRxdxp
x3+a2x2+a3x+a4are called elliptic integrals of the second kind.
Our integral is of thesecond kind.
Elliptic Curves
The equation
is an example of an elliptic curve.
y2 = x(x ¡ 1)(x ¡ (1¡ k2))
0 1¡ k2 1
One can write theequation of such a curve asy2 = 4x3 ¡ ax ¡ b.
Elliptic Curves over C
Let L bea latticeof rank 2 over R.
This means that L = Z! 1 ©Z! 2
We attach the Weierstrass }-function to L:
}(z) = 1z2 +
P06=! 2L
³1
(z¡ ! )2 ¡ 1! 2
´
! 1
! 2
The} function is doubly periodic:This means}(z) = }(z + ! 1) = }(z + ! 2).
e2¼i = 1Theexponential function is periodic sinceez = ez+2¼i :The periods of the exponential functionconsist of multiples of 2¼i.
That is, the period \ lattice; ;
is of the form Z(2¼i).
The Weierstrass function
This means that (}(z);};(z))is a point on the curvey2 = 4x3¡ g2x¡ g3:
The }-function satis̄ esthe following di®erential equation:(};(z))2 = 4}(z)3¡ g2}(z)¡ g3;whereg2 = 60
P06=! 2L ! ¡ 4
andg3 = 140
P06=! 2L ! ¡ 6:
The Uniformization Theorem
Conversely, every complex point on the curvey2 = 4x3 ¡ g2x ¡ g3is of the form (}(z);};(z))for some z 2 C.
Given any g2;g3 2 C,there is a } function such that(};(z))2 = 4}(z)3¡ g2}(z)¡ g3:
Even and Odd Functions
Recall that a function f is even iff (z) = f (¡ z).For example, z2 andcosz are even functions.
A function is called odd iff (z) = ¡ f (¡ z).For example, z andsinz are odd functions.
}(z) is even since }(z) = 1z2 +
P06=! 2L
³1
(z¡ ! )2 ¡ 1! 2
´
};(z) is odd since };(z) = ¡ 2z3 ¡ 2
P06=! 2L
³1
(z¡ ! )3
´.
Note that };(¡ ! =2) = ¡ };(! =2)
This means };(! =2) = 0:
In particular};(! 1=2) = 0};(! 2=2) = 0 and};((! 1 + ! 2)=2) = 0.
This means the numbers}(! 1=2);}(! 2=2);}((! 1 + ! 2)=2)are roots of the cubic4x3¡ g2x¡ g3 = 0:
One can show these roots are distinct.In particular, if g2;g3 are algebraic,the roots are algebraic.
Schneider’s Theorem
If g2;g3 arealgebraicand } is theassociatedWeierstrass }-function, thenfor ®algebraic,}(®) is transcendental.
This is theelliptic analog of theHermite-Lindemann theoremthat says if ®is a non-zero algebraic number,then e® is transcendental.
Note that weget ¼transcendental by setting ®= 2¼i
Since}(! 1=2),}(! 2=2) arealgebraicIt follows that theperiods! 1;! 2 mustbe transcendental wheng2;g3 arealgebraic.
Why should this interest us?
Let y = sinx.Then(dy
dx )2 + y2 = 1.
Thus dyp1¡ y2
= dx.
Integrating both sides, we getRsin b0
dyp1¡ y2
= b.
Let y = }(x).Then³
dydx
´2= 4y3 ¡ g2y ¡ g3.
Thus dyp4y3 ¡ g2y¡ g3
= dx:
Integrating both sides, we getR}((! 1+! 2)=2)}(! 1=2)
dyp4y3¡ g2y¡ g3
= ! 22
Let’s look at our formula for the circumference of an ellipse again.
R11¡ k2
tdtpt(t¡ 1)(t¡ (1¡ k2))
wherek2 = 1¡ b2
a2 :The cubic in the integrand is not in Weierstrass form.
It can be put in this form.But let us look at the case k = 1=
p2.
Putting t = s + 1=2, the integral becomesR1=2
02s+1p4s3¡ s
ds:
The integralR1=20
dsp4s3¡ s
is a period of theelliptic curvey2 = 4x3¡ x:
But what aboutR1=20
sdsp4s3¡ s
?
Let us look at the Weierstrass ³-function:
³(z) = 1z +
P06=! 2L
³1
z¡ ! + 1! + z
! 2
´.
Observe that ³ ; (z) = ¡ }(z).
is not periodic!
• It is “quasi-periodic’’.
• What does this mean?
³(z + ! ) = ³(z) + ´(! ):
Put z = ¡ ! =2 to get³(! =2) = ³(¡ ! =2) + ´(! ).Since ³ is an odd function, we get´(! ) = 2³(! =2).
This is called a quasi-period.
What are these quasi-periods?Observe that³(! 1=2) = ³(¡ ! 1=2+! 1) = ³(¡ ! 1=2) +´(! 1)But ³ is odd, so ³(¡ ! 1=2) = ¡ ³(! 1=2) so that2³(! 1=2) = ´(! 1):
Similarly, 2³(! 2=2) = ´(! 2)
Since ´ is a linear function on the period lattice,we get2³((! 1 + ! 2)=2) = ´(! 1) + ´(! 2).
Thusd³ = ¡ }(z)dz.Recall thatd} =
p4}(z)3 ¡ g2}(z) ¡ g3dz:
Hence,d³ = ¡ }(z)d}p
4}(z)3¡ g2}(z)¡ g3
Hence, our original integral is a sum of aperiod and a quasi-period.
Wecan integrateboth sidesfrom z = ! 1=2to z = (! 1 +! 2)=2 to get´2 = 2
Re2
e1
xdxpx3¡ g2x¡ g3
wheree1 = }(! 1) ande2 = }((! 1 +! 2)=2):
Are there triply periodic functions?
• In 1835, Jacobi proved that such functions of a single variable do not exist.
• Abel and Jacobi constructed a function of two variables with four periods giving the first example of an abelian variety of dimension 2.
1804-1851
1802 - 1829
What exactly is a period?
• These are the values of absolutely convergent integrals of algebraic functions with algebraic coefficients defined by domains in Rn given by polynomial inequalities with algebraic coefficients.
• For example is a period.
¼=RR
x2+y2 · 1dxdy
Some unanswered questions
• Is e a period?
• Probably not.
• Is 1/ a period?
• Probably not.
• The set of periods P is countable but no one has yet given an explicit example of a number not in P.
NµZµQµAµP