Algebraic Geometry Video Lectures

Lectures on Algebraic Geometry from ELGA 2011: Lecture Videos Intersection Theory by Joe Harris: Cycles, Intersection Product, Chow Ring, Grassmanian, Lines on Surfaces, Chern Classes, 5-conic problem, Parameter spaces, Excess intersection. Lectures on Deformation Theory by Robin Hartshorne Lectures on Tropical Geometry by Diane Maclagan Lectures on Algebraic Codes by Peter Beelen

Bombieri, Friedlander, Iwaniec

Primes in Arithmetic Progressions to Large Moduli. https://projecteuclid.org/journals/acta-mathematica/volume-156/issue-none/Primes-in-arithmetic-progressions-to-large-moduli/10.1007/BF02399204.full Primes in Arithmetic Progressions to Large Moduli. II. https://eudml.org/doc/164255Primes in Arithmetic Progressions to Large Moduli. III https://www.ams.org/journals/jams/1989-02-02/S0894-0347-1989-0976723-6/ Dipsersion, sums of Kloosterman sums

Language Models -1

Imagine we have a corpus representing a text in English language and we use it for training data. And we would like to assign probabilities to a given document of how likely is it to be a valid english text. A Language model assigns a probability to the document by assuming a probability distribution on […]

Tannaka-Krein Duality, Pontryagin Duality

Consider finite abelian groups. We know their character theory which allows us to do harmonic analysis on these groups. The characters allow us to capture all the information about the group. In fact, we explicitly have that natural the character-group element pairing gives an isomorphism . There are several ways to read this isomorphism. For […]

Diophantine Sets

Diophantine sets are subsets defined by polynomial equations over integers. More precisely We obtain them basically by projecting integers solutions of some polynomial equations. Observation 1: Using the statement iff we see that there is no difference between one polynomial equation and a system of polynomial equations. Example: Even numbers are Diophantine: Odd numbers: Non-negative […]

Selberg’s Sieve

From a set , assume that we want to pick elements that are not divisible by any primes in a set . Let be the set of element of divisible by If we write the inclusion-exclusion to sift out the element divisible by some prime in , we get Even if we know very accurately, […]

Functorial way, equidistribution

I am almost always surprised and excited to find functorial changes and dualities turning the problems that look hard in one category into something manageable. Here is a puzzle that I came across recently. It’s called the rectangle tiling theorem and says that any rectangle that is tiled with rectangles with at least one integer […]

Introduction to Algebraic K theory

We shall start looking at some algebraic K theory. There are several perspectives on this topic. The theory is motivated by ideas in several areas like homological algebra, the theory of vector bundles, homotopy theory, etc. There are some constructions like group completions (symmetric monoids), plus constructions ( rings), Q-constructions (exact categories), Waldhausen constructions (categories […]

Fermat’s quotients

For a prime and an integer the Fermat quotient is defined as Let be the smallest valueof for which We want to understand how looks. For instance, Granville conjectured Lenstra conjectures Lenstra proved Granville showed that for . How do we get upper bounds on ? Main ideas: (Smooth numbers+ Equidistribution of multiplicative subgroups) If […]

Legendre Transform

Legendre transform is a duality concept that comes up in building the dual theory of Hamiltonian mechanics from Lagrangian mechanics.  Legendre transform of a function on a space gives a function on the dual space. In mechanics, for instance, it gives the hamiltonian of the cotangent phase space from the lagrangian of the configuration space( […]

Hecke Operators

What are Hecke Operators? These are some operators acting on automorphic forms on some arithmetic groups. Not all discrete subgroups have these operators and existence of these operators is a reason for why the spectrum of these group is different from a generic spectrum and has different statistics compared to the universal distributions expected. It’s […]

Siegel Half-Space and Modular Forms

Siegel Modular Forms: Consider the Siegel upper-half space defined by where are symmetric real matrices, is a positive definite matrix. Convex combinations of psd matrices are positive define, hence this is a convex subset of the subspace of symmetric matrices. For example gives the usual upper-halfplane . We have the action of on by Mobius […]

Dynamics, Ratner’s theorems, Rigidity

Gorodnik’s lecture notes on Dynamical systems in Number theory:https://www.math.uzh.ch/gorodnik/dyn_num/index.html Dave Morris- Ratner’s Book: http://people.uleth.ca/~dave.morris/books/Ratner.pdf Lindenstrauss- Dynamics and Number Theory: http://www.ma.huji.ac.il/~elon/Publications/ICM2010.pdf Akshay Venkatesh- Littlewood Conjecturehttps://www.math.ias.edu/~akshay/research/eklexp.pdf Dmitry Kleinbock- Application of Homogenous dynamicshttps://arxiv.org/pdf/math/0210301.pdf ADDED LATER: MSRI Workshop https://www.msri.org/workshops/742/schedules/19308 Dave Morris Slides https://docs.google.com/viewer?url=https://www.msri.org/workshops/742/schedules/19303/documents/2445/assets/23238

27 lines on a Cubic Surface

Theorem: On every smooth cubic surface in , there are exactly 27 lines. How do we prove such a result? Proof Sketch/Ideas: Find a nice cubic surface where you can exactly compute all the lines on the surface. Show that this count doesn’t change under deformations. And that the space of smooth cubic surface is […]

Negative Pell’s Equation

We want to understand how many discriminants D have solutions to negative Pell’s equation. First of all we have some local restrictions on the the prime factors of D if there is a solution. There cannot be prime factors 3 modulo 4. If we restrict to these prime factors for D, (special discriminants) how likely […]

Squarefull Numbers

A positive integer is squarefull if each prime in the factorization occurs at least twice. We want to count the number of squarefull integers less than for a large . In general, numbers with each prime occurring at least times are called powerfull numbers. The indicator functions of powerfull numbers are multiplicative functions. Erdos Szekeres […]

Large Sieve and Hilbert Inequalities

Large Sieve Inequalities are very powerful tools in analytic number theory to show cancellation in sums of harmonics twisted with coefficients that we know little about or hard to deal with (imagine non-smooth arithmetic coefficients). By regarding coefficients as general unknown coefficients, we can still obtain cancellation when we average over a set of “almost” […]

Peter Gustav Lejeune Dirichlet

Dirichlet made deep contributions to mathematics, especially in number theory by his work on primes in arithmetic progressions, introducing Characters and L-functions, Class number formula, Units in number fields, Divisor problem, Diophantine approximations, reciprocity laws, special cases of Fermat’s last theorem. In analysis he introduced important ideas like Dirichlet Kernel (Fourier convergence), Dirichlet boundary conditions, […]

Poisson Summation

where Poisson summation is one of the most beautiful and powerful identities in mathematics. It’s a trace-formula that relates a sum over integer lattice to it’s sum over the dual lattice of integers. In general, it relates a sum over a lattice to a sum over the dual lattice. is periodic function , hence defined […]

Bocherds Product for j-function

Consider the function We have This identity is equivalent to some non-trivial polynomials relations in the coefficients of the function. For example Proof: Apply Hecke operator on to get Let us say is the Fourier expansion for We can compute and see that So we can observe the only negative coefficients that appear in is […]

Rational Points on a Sphere

Say we want to count rational points on the unit sphere of a bounded height and study their distribution. Counting: The number of rational points on the sphere of height bounded by is given by where The rational points of height correspond to primitive integer points on the sphere of radius by clearing the denominators. […]

Computation of Riemann Zeta function

Euler-Maclaurin type formula with Bernoulli numbers One can accelerate the convergence of the Dirichlet series by acceleration methods to compute zeta function for small imaginary parts. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.56.9455&rep=rep1&type=pdf Poisson summation to get Riemann Siegel formula. Another way to see this is approximate functional equation. So now we are reduced to computing exponential sums . Using Taylor […]

Quasi-Modular Forms, E_2

Consider holomorphic modular forms and take their derivatives. The resulting functions are not modular. In fact, let Take any weight modular form , that is a holomorphic function satisfying and is holomorphic at infinity, hence looks like We have the following formula for it’s derivatives So instead of just the term , we have extra […]

Euler’s formula,planar graphs

In  this post I will describe certain applications of Euler’s formula for planar graphs. Euler’s formula states that for a planar graph. Thus acts a invariant for planar graph. Infact, more generally euler’s characteristic is a topological invariant of certain objects like simplical complexes. So a direct application euler’s formula shows that graphs and are […]

Constructive Proof of Chernoff Bounds

Chernoff bounds allow us to get the concentration of sum of independent binary random variables. (We can relax the independence to negative correlation/association etc). The standard proof goes by considering the idea of Bernstein to use moment generating function which allows us to decouple the variables. That is using the fact that exponentials behave nicely […]

The equation $A^4+B^4+C^4=D^4$

Elkies found a solution giving a counter examples to Euler’s conjecture that there are no solutions in positive integers to , the generalizations of Fermat’s equations to multiple variables . In the case , a counterexample was found by computer search, but was much harder, and only after Elkies found the above solution, Frye found […]

Non-vanishing of L(1, χ)

To prove Dirichlet’s theorem for primes in arithmetic progressions consider the weighted sums over primes in an AP and sums over primes twisted by characters. and We have the following result which relates the L-values to primes. Proof: Use and evaluate Note that are bounded quantities, hence is bounded provided In the above calculation, if […]

Number of imaginary fields with a given class number

The class number formula shows that is roughly Thus studying the distribution of class numbers can be related to the distribution of as varies. https://arxiv.org/pdf/0707.0237.pdf – Soundararajan The average value of the class number The distribution of the L values are governed by the statistics of a random Euler product. https://arxiv.org/pdf/math/0206031.pdf – Granville, Soundararajan

Erdos-Turan Inequality

By using harmonic analysis to detect intervals, equidistribution of a sequence modulo one can is equivalent to the following: Weyl’s criterion: A sequence of real numbers is uniformly distributed mod one if and only if for every integer we have That is this criterion gives equidistribution . What is the rate of convergence? Can we […]

Hensel’s Lemma

Hensel’s lemma gives necessary conditions to lift a solution mod of a polynomial equation to a solution mod . Collecting all these solutions mod , we get a solution in the p-adic integers. Basic Version: If an integer polynomial satisfies then there is unique solution to . All these solutions can be put together to […]

Elliptic functions, Addition Formulae

Consider the ellipse with eccentricity given by is called the modulus. (We just assumed the length of the minor axis ) Define to be the “angular” arc-length parameter given by Here are the polar coordinates. We define elliptic functions by the formulae (These can be tough of deformations of and , in fact we get […]

Quaternions and Rotations

Hamiltonian discovered the following Quaternion Algebra over reals. Hamilton’s quaternions is the four dimensional algebra with the relations Here and hence all reals are assumed to commute with every element. This algebra can also be seen as a two dimensional algebra over complex numbers where is satisfies But the complex numbers are not in the […]

Dirac Operators

Dirac operator is a differential operator that is formally a square root of a second order operator like the Laplacian. For example, consider the Laplacian on given by . The Dirac operator here is given by which satisfies . Consider the operator that acts pairs of functions on the Euclidean plane given by Using and […]

Quaternion Algebras

Given non-zero elements and in a field , the quaternion algebra over a field is the 4-dimensional vector space with the relations In characteristic , the quaternion algebra is defined by the relations Any algebra generated by non-zero elements , satisfying these relations with and non-zero has to be the quaternion algebra, that is it […]

Hilbert Inequality

Inequalities of the form and more generally, for , are called Hilbert’s inequalities (Hardy-Hilbert Inequalities). This can be viewed in terms of matrices/operators. Let be defined as We are looking for bounds of the form . By Cauchy-Schwarz, this is equivalent to having Thus we are trying show the operator is bounded and the best […]

Effective Results in Number Theory

Many results are asymptotic in the sense that certain bounds are true for sufficiently large values of the parameters involved. Sometimes, one can determine the explicit bounds for how large we need the quantities to be by carefully working through the estimates and some other times, we just know that there is a finite number […]

Categories, Universal property

In this post I will discuss some basic category theory. I have discussed what a category is, in a previous post. A category basically consists of some objects and maps between these objects are called morphisms. Morphisms are a way to tell which object in our category are related. Now we want the relationship between […]

Riemann Mapping Theorem

Given two simply connected proper open subsets of the complex plane, there is a conformal bijection between them. How do we prove such a result? The result in particular implies that any simply connected proper open set should be biholomorphic to the open unit disk in the complex plane. In fact if we see that […]

Sums of two cubes

How many numbers can be represented as sums of two cubes? How many of them in exactly one way? How many of them in at least ways etc? Counting the the number of points we count the numbers with multiplicity and it turns out to be . (Each of the variables has around choices which […]

Periodic orbits: Bendixson-Dulac

Consider the system How do we get information about the number of periodic orbits of this system? Can we show that there are no periodic orbit given some system? What are some sufficient conditions? Bendixson: On a simply connected domain , if the quantity doesn’t change sign, then the above system has not periodic orbits. […]

Tangent space, Derivative

In the previous post we have seen some examples of manifolds and the most general ways in which they occur. By now we know what we mean by differentiable functions on the differentiable manifolds. But we have not defined what do we mean by derivative of a function. So the the natural way to do […]

Gauss Duplication Theorem

One way to define the genus of quadratic forms is by the values represented by the form or by the complete characters( the values of genus characters). In terms of the class group, forms in the principal genus are exactly the square classes, form in a particular genus form a coset of the square classes. […]

Differentiable structures-II

In the previous post we have defined a differentiable structure.The definition allows us to talk of differentiable functions i.e., we gave meaning to the term ” f is differentiable function on the manifold”. So basically, differentiable structure is the topological manifold together with sets of differentiable functions where the sets satisfy some properties. For instance […]

Lyapunov Functions

How do we prove stability of a critical point?Idea: If we can find a function non-negative function which doesn’t increase with time and vanishes only at the critical point, then we see that the trajectories sufficiently close to the critical point will stay close to the critical point. For consider the system The linearized approximation […]

Non Linear ODE Resources

Arnold, V. I., Ordinary Differential Equations Differential Equations, Dynamical Systems, and an Introduction to Chaos by Morris W. Hirsch, Stephen Smale and Robert L. Devaney Fred Brauer, John A. Nohel – The Qualitative Theory of Ordinary Differential Equations Nonlinear ordinary differential equations by Dominic Jordan, Peter Smith Guckenheimer, J. C., and Holmes, P., Nonlinear Oscillations, […]

Hasse-Minkowski

Local Global principle for quadratic forms over a field. Why is it true? How do the local solutions “force” Global solutions? Let stick to rationals. What happens for variables? This case basically corresponds to understanding the form Having a non-trivial rational solution corresponds to being a square. And this can be checked locally! (Every non-square […]

Construction of Regular 17-gon: Gauss

June 1, 1796 Jenenser Intelligenzblatt : “Every beginner in geometry knows that it is possible to construct different regular polygons [with compass and straightedge], for example triangles, pentagons, 15-gons, and those regular polygons that result from doubling the number of sides of these figures. One had already come this far in Euclid’s time, and it […]

Quadratic Reciprocity

Quadratic reciprocity is one of many deep facts of arithmetic relating different local properties (modulo primes). In this post I try to explain it in terms I understand. I always wanted to write about it, but have been postponing. Theorem: Let be two odd primes.We have Here denotes the Legendre symbol which detects whether is […]

Ramanujan’s Partition Identities, Congruences

We want to prove Ramanujan’s congruence identities for the partition function: We prove them using identities involving series of the generating function: Pentagonal identity: (You can directly prove this or obtain it as consequence of Jacobi -Triple Product identity) Let be a fifth root of unity. Using the following Ramanujan’s identity (obtained from specializations of […]

Lie Theory-Symmetry of Differential Equations

Consider the differential equation This equation can be solved in integration (quadrature) and every solution is obtained by shifting one particular solution.(Constants of integration)Is it possible to change coordinates in the first equation and reduce to the second case where we can solve equation explicitly.One way to view the reason for this solvability is the […]

PNT implies Mobius cancellation.

Let us assume we have How do we prove the following? Here is how a proof goes: Using the identity and comparing the Dirichlet coefficients, we get Now summing over , Using PNT for sufficiently large and the Chebyshev bound for smaller , we get Replacing by introduced an error of because therefore and cancelling […]

Mordell’s Equation

The equation is called Mordell’s equation. For , it is non-singular and defines an elliptic curve (genus 1, group structure). Rational Points: We first describe the torsion part of the rational solutions. By Nagell-Lutz, these solutions should have integer coordinates. Torsion: Assume that is free of sixth powers. If not we can use the change […]

Mazur-Ulam theorem

If we have map between two Banach spaces which preserves distances, then it has to be a linear! (Affine) Let be the map. To prove that is affine it is sufficient to show that Consider the defect which is bounded by Construct another isometry where is a reflection in the Y space about the point […]

Szemeredi’s Regularity Lemma

The goal is to partition a dense graph into few pseudorandom parts. Edge density -regularity: For a pair of vertex sets and is called -regular, if for all subsets satisfying we have Regular partition: A partition of into sets is called an -regular partition if A way to achieve such partitions is to take: with1. […]

Theory of Sorting

I have been interested in sorting algorithms and structures since a long time. The simple looking theory has lot of rich and beautiful structure. Let us see what are the most natural ideas for sort. First of all given a sequence of elements, we use some properties of these elements to order these elements. The […]

Sophie Germain’s Theorem

As early attempts to the proof of Fermat’s last theorem, many mathematicians solved the problem for small exponents. While these special cases are being studied, Sophie Germain, a French mathematician, came up with the following interesting result. (Look at https://www.agnesscott.edu/lriddle/women/germain.htm for her fascinating and revolutionary story) Theorem 1: For any odd prime such that is […]

Gaps between Farey fractions

Consider the rationals (fractions) between and with denominator bounded by . That is These fractions are called Farey fractions of level and we denote them by The number of fractions is Neighbours: Any two neighbours have the property that In fact, two fractions are adjacent to each other in some Farey sequence iff . In […]

Erdos-Selberg Elementary PNT

Elementary Prime number Theorem1. Selberg symmetry identity : Proof: 2. Tauberian argument Iterative arguments- Using Brun-Titchmarsh Relation to the analytic proof and non-vanishing of zeta(s) on Where are the zeroes?Smoothing of mobius- to get these higher Von Mangoldt functions.Relation to Chebyshev’s estimates? Approximations to mobius.. Error terms in this elementary proof- use generalized Selberg identities […]

BLR Linearity testing

A function is linear if either of the following conditions hold: for some (Global description)For all (Local description) What can we say about function which satisfies many of these local requirements? How close is it to a linear function? How does the distance to linear functions related to the fraction of local requirements satisfied? BLR […]

Isoperimetric Inequality

Minimum length of the curve bounding an area A? Maximum area bound by a closed curve of length L? Optimal case: Circle. Steiner Proof: Consider a curve with given perimeter 1. If the region bound by the curve is not convex – there is a chord joining two point on the curve which lies outside […]

Groups, Categories, Representations

A way to think of groups is that they correspond to symmetries. Symmetries of an object satisfy some properties which exactly correspond to the group axioms. An equivalence between an object and its other transformed form is what we mean by symmetry. Act of doing nothing to the object does not transform the object. It […]

Peg Solitaire Invariants

The peg solitaire (Hi-Q) is a game defined by moves where one can move a peg orthogonally over an adjacent peg to an empty peg while removing the peg that’s jumped over. One can ask many question about the game, about the possible states that can be reached from a given initial state or about […]

Legendre, Jacobi, Kronecker Symbols

For an odd prime , the Legendre symbol is defined as the quadratic residue symbol, This is a character modulo and is helpful as a “harmonic” in contrast to the Gauss’s notation and which serve as indicators for being a quadratic residue and non-residue. We also distinguish and rest of the quadratic residues. Some properties […]

Sums of Squares – Continued Fractions

Given a prime we want to find solutions to Method 1: Find the solution of where Expand into a simple continued fraction to the point where the denominators of its convergents satisfy the inequality Then Proof: For any , we have which is true for any continued fraction expansion. Let Take We have Therefore Method […]

Quadratic Irrationals, Continued Fractions, Pell’s Equation..

Quadratic irrational are solutions to quadratic equations with integer coefficients. Continued fractions: Expansion of the form If then Relation to GL_2 (invertible integers matrices): Quadratic Irrationals and periodic continued fractions: Theorem: An infinite integral continued fraction is periodic if and only if it represents a quadratic irrational. Proof: (Easy) Multiplying it out, we see satisfies […]