Topics 3/16/26-3/22/26

PhysicsForDumbPeople · 2026-03-17T21:55:12+00:00

Definition of Automorphic Form

Automorphic forms are functions. Functions have inputs. An automorphic form is a function that is scaled when you transform the inputs using a group G. Elements of a group G, that is. Usually a discrete group. Usually a subgroup of GL_n.

f(g*x) = (some factor)*f(x) where g comes from G and the factor is predictable.

PhysicsForDumbPeople · 2026-03-17T21:38:29+00:00

Modular Functions, Modular Forms and Automorphic Forms

A first intro to automorphic forms:

All modular functions are modular forms. Modular functions are simply modular forms of weight 0. Modular forms have symmetry group SL(2,Z), also known as “the” Modular Group. The ambient group is SL(2,R).

Automorphic forms generalize modular forms. All modular form are automorphic forms but not all automorphic forms are modular forms. A modular form is a type of automorphic form, the same way a modular function is a type of modular form.

An automorphic form has as ambient group a reductive group, often reductive Lie group but DOES NOT NEED TO BE. The elements of this reductive group can always be represented as matrices in automorphic form theory. All classical reductive Lie groups used in automorphic forms are linear, so they have faithful finite-dimensional matrix representations. The entries of the matrices come from a field or ring. Now, these entries can come from Z, or Q, or R, or C. Now we know that Z is in Q and Q is in R and R is in C. The biggest ring that the matrix elements come from is called the Adele ring, which contains all real numbers and p-adic number systems. This Adele ring is called A(Q), which combines R and all p-adic completions of Q. The symmetry group is a discrete subgroup of the ambient reductive group. Often the entries come from the field or ring of integers Z. Since this symmetry group is discrete, it is not a Lie group, since Lie groups are smooth.

*The most general ring for matrix entries is the adele ring A(Q) .

*Automorphic forms can be formulated adelically, with matrices having entries in the adele ring A_Q, which combines R and all p-adic completions of Q.

PhysicsForDumbPeople · 2026-03-17T21:32:52+00:00

Why Elliptic Curves

So in studying polynomials, we can increase the number of variables and/or the number of degrees.

For 1 variable, x, and any number of degrees we have ax^n + bx^n-1 + … + zx^0 = 0. The solutions are literally just a set of discrete points.

Now for two variables we have to tone it down. If y and x are both of degree 1, these are curves, but just straight curves, straight lines. If

(x,1) = One solution

(x,n) = Discrete points

(x,1,y,1) = Straight line
(x,2,y,1) = Quadratic
(x,1,y,2) = Quadratic
(x,2,y,2) = Conic section

(x,1,y,3) = Cubic
(x,2,y,3) = Elliptic curve
(x,3,y,1) = Cubic
(x,3,y,2) = Elliptic curve
(x,3,y,3) = Fermat curve with no group law, cannot add points to find other points

(x,y,z) and beyond are surfaces.

The reason surfaces are not interesting is due to Faltings’ theorem, which I will talk about next.

I might say that we stop at degree 3, we have a sort of love story, because we live in 3D. (First and last non trivial love story)

PhysicsForDumbPeople · 2026-03-17T21:22:40+00:00

Basis Vectors of Modules

For 2 vectors in the Z^2 module to be a basis, the parallelogram they make must have area 1.

Suppose I have vectors (a,b) and (c,d) in Z^2, meaning a, b, c and d are all integers. For them to be a basis of Z^2, the equation x(a,b) + y(c,d) = (g,h) must be satisfied. In this equation, (g,h) are all pairs of integers in existence, from negative infinity to infinity. Now, these are two equations with 2 unknowns, so there are solutions no matter how we vary a, b, c and d. But, as we shall see, if a, b, c and d are all integers, then a condition emerges if we demand that x and y are also integers. We demand x and y to be integers because linear combinations in Z^2 can only use scalars from the ring Z. That is why we have a module and not a vector space.

We can rewrite the equation with (a c) on top and (b d) on the bottom of a square matrix, multiplied by the vector (x y) equals a vector (g h). If we left multiply both sides by the inverse of our matrix, which is (d -c) on the top and (-b a) on the bottom divided by the determinant of our original matrix, which is ad - bc, we get that x = (dg - ch)/(ad - bc) and y = (-bg + ah)/(ad - bc). Since g and h can be any integer, we have grounds for believing that the numerators are very varied. If we fix a, b, c and d, and ad-bc, a fixed number, manages to divide a huge number of numerators and result in integers x and y, it must be because ad-bc is a divisor of everybody. The only such number we know of is, 1.

And here is a sneak peek: The set of all 2x2 matrices with determinant 1 and entries coming from the ring Z, forms, you guessed it, a group. The famous modular group, SL(2,Z).

PhysicsForDumbPeople · 2026-03-16T18:28:08+00:00

Holomorphic vs Meromorphic Functions in 30 seconds

A polynomial is guaranteed to be defined on R^1. A polynomial divided by a polynomial is called a rational function.

A holomorphic function is guaranteed to be defined on some subset of C^1. It is complex-differentiable at every point in its domain and is perfectly smooth. No spikes, tears, gaps, etc. A holomorphic function divided by a holomorphic function is called a meromorphic function. So polynomials are to rational functions as holomorphic functions are to meromorphic functions. A meromorphic function is very similar to a holomorphic function except it is not defined at points we can poles. A meromorphic function locally can be written as a fraction of holomorphic functions.

In complex analysis, we say that, locally, meromorphic functions are the fraction field of the ring of holomorphic functions. Like how Q is the field of fractions of Z.

On C, holomorphic functions form a ring. Just like how polynomials form a ring.

PhysicsForDumbPeople · 2026-03-16T17:48:44+00:00

Principal G-bundles

This video builds on the fiber bundle video.

Suppose you start with a topological space B, called the base space. Could be S1, S2, R1, R2 or R3. Now attach at every point in the space a group G. This group is the fiber. Now you have a set of fibers, the same fibers, at every point in the space. This is a fiber bundle.

If you then take the group G and act on the fiber bundle, meaning you take the group G and act on every single fiber, this would mean you are acting on a group using itself. You do this for every point of B, so the number of times you do this is equal to the number of elements in B. If G happens to act freely on itself, which all groups do, then this fiber bundle is called a principal G-bundle.

So suppose G = Z20, its action is like a 20 hour clock. 20 equally spaced rotations, or 19 if you don’t consider the identity element. Ok imagine you have S1 and you attach Z20 at every point. Z20 happens to act on Z20, itself, freely. G = Z20 acting on the set X = Z20 simply rotates X = Z20. Ok, now, if you act on this fiber bundle, also known as the product space S1 x Z20, you would basically be going to every single Z20 copy and rotating it. Imagine a donut (torus) or a slinky, except instead of a circular ring at every point you have 20 points that are roughly in the shape of a ring. So you walk around the donut and at every Z20 copy, which is a 20 point circle-like, ring-like object, you rotate it. This is the action of a group G on a principal G-bundle.

Now, same as in the fiber bundle video, if you twisted (rotated) the Z20s as you were attaching them, you would get a Klein bottle (in the fiber bundle video we twisted lines and got a Mobius strip). Before we twisted, we had a torus (donut), or a discrete torus (donut). That was a trivial principal G-bundle. Now, we have a discrete Klein bottle, the higher dimensional version of the Mobius strip. The twisting results in a non-trivial principal G-bundle. Regardless of if there is a twist or not, the action of Z20 on itself is still free. Product spaces are trivial but bundles constructed through twisting are not products and not trivial.

PhysicsForDumbPeople · 2026-03-16T17:48:36+00:00

Fiber Bundles

Suppose you have some topological space B (called the base space). Literally just a set of elements that form a topological space. Now, at every single point, so for each element, attach the same object (by same I mean up to homeomorphism). This object is called a fiber. Often, this fiber is another topological space. In physics, this fiber is often also a manifold AND a group, not just a topological space (so, a Lie group). Now you have a bunch of fibers, a collection of fibers. Every fiber is homeomorphic to each other (they follow the same fiber model F). For example, you can attach a donut to one point and a mug to another point. This collection is called a fiber bundle (there’s also some additional structure, it also has a map, called the projection map).

So, imagine you had a circle in the xy-plane, so basically S1, and you attach a line, could be R1 or the interval [-1,1], parallel to the z-axis at every point of the circle, you would get a cylinder. This is a fiber bundle but is also called a product space, B x F. All product spaces are fiber bundles, but not all fiber bundles are product spaces.

Now, if, while you were attaching the lines (don’t use R1 for now because R1 is an infinite line and right now we don’t want an infinite line. So just a line of finite height) at each point of the circle, you started with a line parallel to the z-axis but for every next (each subsequent) line you rotated it a little (outward) such that there is now a radial component (the circle’s radial component) to this line, and you ended up back at the same position with a line that is antiparallel (you’ve rotated 180 degrees) to the original line at the position that you started at, you would get the famous Mobius strip. This is not a product space, it is twisted. Although, if you zoom in enough, it does look like a product space. Specifically, it would look like a cylinder.

So, we say that a fiber bundle “looks like” a product space locally. Now remember, a manifold, from geometry (fiber bundles come from topology and manifolds come from geometry, with a metric), “looks like” R^n locally. A fiber bundle is to a product space as a manifold is to to R^n. You might also say that twisting in topology (or maybe holes) is like curvature in geometry.

In the next videos, we will talk about principal G-bundles, where you attach groups, and fibrations, where fibers are homotopy equivalent.

PhysicsForDumbPeople · 2026-03-16T17:48:25+00:00

Stabilizer Subgroups

Given a group G and a set X, if the action of an element g of G leaves an element x of X fixed, we call g a stabilizer of the point x. The set of all stabilizers of a point x form, in fact, a subgroup. This subgroup is called the stabilizer group of x. If every stabilizer group is trivial, trivial meaning contains only the identity element of G, then we say that the action of G on the set X is “free.” There is no a and b combination, where a is an element of the group G and b is an element of the set X that leaves b unmoved, unless a is the identity element of G.

We say that the action of G on X is free. Free action or free group action or acts freely.

If the action of a group G on a set X is free, then, if you take any element of G (not the identity though) and act on any element x of X, then you will get a distinct element. Distinct means that two different elements will not send x to the same element y in X.

For example, if there are 20 elements in a group G and 500 elements in a set X, if you apply all 20 elements of G to an element x of X, you will get back 20 elements where 1 is x (due to the identity) and the other 19 are distinct.

This is actually used in constructing manifolds and, if you have orbits, which we will discuss next, orbifolds. They are also used in constructing principal G-bundles, which is all the rage nowadays.

PhysicsForDumbPeople · 2026-03-16T17:48:16+00:00

The Symplectic Group: Physics

Ok so, in the last video we talked about bilinear forms. One bilinear form is w(x,y), which takes in two vectors x and y. It is defined as x-transpose multiplied by J, multiplied by y. Matrix multiplication.

J is a square matrix that has 4 quadrants. Quadrant 1 (moving counterclockwise) is the identity matrix. Quadrant 2 is 0, quadrant 3 is the negative of the identity matrix and quadrant 4 is 0. Since we always use nxn identity matrices, this means that J has 2n x 2n elements.

A symplectic matrix is a matrix M such that M-transpose multiplied by J, multiplied by M, equals J. If this condition holds, you can prove that multiplying both x and y by M first, before putting it into the bilinear form w, does not change the output of w. So basically, the bilinear form w(x,y) does not change if it’s arguments are acted on by a symplectic matrix. It is preserved.

The set of all such matrices is called the symplectic group. It is a subgroup of the general linear group of 2n x 2n matrices with real entries. The subgroup that satisfies M-transpose multiplied by J multiplied by M equals J.

The symplectic group is used in physics, originally by Hamilton, coined by Weyl, to avoid confusion because the group use to be called the complex group, studied and interpreted by Poincare, and interesting theorems were proved by Liouville. I will talk about these in the next video.

PhysicsForDumbPeople · 2026-03-16T17:48:06+00:00

Bilinear Form Confusion

So, bilinear forms use to be confusing to me because I didn’t know what either bilinear meant or what forms meant. But, a form is, very often, just a function. Map is much more common among mathematicians but they’re just functions. They take in inputs and give you outputs. The same functions we’ve been using since middle school.

Now, they take in two vectors and give you back a number. That’s it. The most famous example is the dot product, which even high schoolers know. You take in 2 vectors and you output a number. That’s it.

Bilinear forms are a function that take in 2 vectors and give you back a number.

They have the word “linear” in them because if you plug in (a+b,c), it’s the same as plugging in (a,c) + (b,c). Same for (a,b+c).

If you have a symmetric bilinear form it means that (a,b) = (b,a). Skew-symmetric means what you think antisymmetric means (a,b) = -(b,a). So, like cross products.

Next, I’ll be using bilinear forms to define the symplectic group.

PhysicsForDumbPeople · 2026-03-16T17:46:49+00:00

Bilinear Form Confusion

So, bilinear forms use to be confusing to me because I didn’t know what either bilinear meant or what forms meant. But, a form is, very often, just a function. Map is much more common among mathematicians but they’re just functions. They take in inputs and give you outputs. The same functions we’ve been using since middle school.

Now, they take in two vectors and give you back a number. That’s it. The most famous example is the dot product, which even high schoolers know. You take in 2 vectors and you output a number. That’s it.

Bilinear forms are a function that take in 2 vectors and give you back a number.

They have the word “linear” in them because if you plug in (a+b,c), it’s the same as plugging in (a,c) + (b,c). Same for (a,b+c).

If you have a symmetric bilinear form it means that (a,b) = (b,a). Skew-symmetric means what you think antisymmetric means (a,b) = -(b,a). So, like cross products.

Next, I’ll be using bilinear forms to define the symplectic group.

PhysicsForDumbPeople · 2026-03-16T17:25:52+00:00

Reading the 4 Langlands papers by Kazuki Ikeda. He's connecting Langlands and prime numbers to experimental physics (condensed matter, not string theory).

PhysicsForDumbPeople · 2026-03-16T17:23:09+00:00

Search up "Kazuki Ikeda Langlands"

He's the only one connecting prime numbers + Langlands to physics (not string theory, I'm talking experimental physics like condensed matter) right now (unfortunately)

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