Bring the notes up to date with the latest prototype, commit 9d69a9

Gram-matrix-parameterization.md

@@ -2,36 +2,65 @@
 
 ### Construction elements as vectors
 
-Take a 5d vector space $V$ with a bilinear form $(\_\!\_, \_\!\_)$ of signature $-++++$, which we'll call the *Lorentz form*. In [inversive coordinates](https://code.studioinfinity.org/glen/dyna3/src/branch/main/notes/inversive.md), points and generalized spheres are represented, respectively, by timelike and spacelike vectors in $V$. If we normalize these vectors to pseudo-length $\pm 1$, and choose a vector on the lightlike 1d subspace representing the point at infinity, a lot of the constraints we care about can be expressed by fixing the Lorentz products between vectors.
+Take a 5d vector space $V$ with a bilinear form $(\_\!\_, \_\!\_)$ of signature $++++-$, which we'll call the *Lorentz form*. In [inversive coordinates](https://code.studioinfinity.org/glen/dyna3/src/branch/main/notes/inversive.md), points and generalized spheres are represented, respectively, by timelike and spacelike vectors in $V$. If we normalize these vectors to pseudo-length $\pm 1$, and choose a vector on the lightlike 1d subspace representing the point at infinity, a lot of the constraints we care about can be expressed by fixing the Lorentz products between vectors.
 
 ### Constraints as Gram matrix entries
 
-The vectors $a_1, \ldots, a_n \in V$ representing the elements of our construction can be encoded in the linear map $A \colon \mathbb{R}^n \to V$ with $Ae_j = a_j$ for each of the standard basis vectors $e_1, \ldots, e_n$. We can then express constraints by fixing elements of the Gram matrix $G = A^\top A$, where $\top$ is the adjoint with respect to the inner product $\langle\_\!\_, \_\!\_\rangle$ on $\mathbb{R}^n$ and the Lorentz form. Since the inner product and the Lorentz form are both non-degenerate, the rank of $G$ matches the dimension of the span of $a_1, \ldots, a_n$.
+The vectors $a_1, \ldots, a_n \in V$ representing the elements of our construction can be encoded in the linear map $A \colon \mathbb{R}^n \to V$ with $Ae_j = a_j$ for each of the standard basis vectors $e_1, \ldots, e_n$. We can then express constraints by fixing elements of the Gram matrix $G = A^\dagger A$, where $\dagger$ is the adjoint with respect to the inner product $\langle\_\!\_, \_\!\_\rangle$ on $\mathbb{R}^n$ and the Lorentz form. Since the inner product and the Lorentz form are both non-degenerate, the rank of $G$ matches the dimension of the span of $a_1, \ldots, a_n$.
 
 The symmetric bilinear form $\langle\_\!\_, G\_\!\_\rangle$ is the pullback of the Lorentz form along $A$:
-\[\begin{align*} \langle\_\!\_, G\_\!\_\rangle & = \langle\_\!\_, A^\top A\_\!\_\rangle \\ & = (A\_\!\_, A\_\!\_). \end{align*}\]
+\[\begin{align*} \langle\_\!\_, G\_\!\_\rangle & = \langle\_\!\_, A^\dagger A\_\!\_\rangle \\ & = (A\_\!\_, A\_\!\_). \end{align*}\]
 
-## Reconstruction
+### Coordinate computations
 
-### Assumptions
+When we choose a basis for $V$, represented as an identification $V \cong \mathbb{R}^5$, the Lorentz form gets encoded in the matrix $Q \colon \mathbb{R}^5 \to \mathbb{R}^5$ defined by the relation $(\_\!\_, \_\!\_) = \langle\_\!\_, Q\_\!\_\rangle$. Using the fact that $A^\dagger = A^\top Q$, where $^\top$ is the adjoint with respect to the inner product $\langle\_\!\_, \_\!\_\rangle$ on both $\mathbb{R}^n$ and $V \cong \mathbb{R}^5$, we can express the Gram matrix $G = A^\top Q A$ in terms of positive-signature matrix operations. This expression is convenient for programming, because linear algebra packages tend to be written for positive signature.
 
-Throughout this section, we'll assume $a_1, \ldots, a_n$ span $V$, so $G$ has rank five.
+## Reconstructing a construction from its Gram matrix
 
-### Reconstruction from the Gram matrix
+### Reconstruction as low-rank factorization
 
-Write $G$ as $LU$, where the matrix $U$ is a row echelon form of $G$, and the the matrix $L$ encodes the sequence of row operations that restores $G$. Since $G$ has rank five, the first five rows of $U$ are non-zero, and the rest are zero. Thus, the first five columns of $L$ are non-zero, and the rest are zero. Since $G$ is symmetric, we should have $L = U^\top D$, where $D$ is a diagonal matrix that has one negative entry and four positive entries at the pivot positions on the diagonal.
+#### Overview
 
-The first five rows of $U$ form the matrix of $A$ in some orthogonal basis for $V$. The matrix $D$ describes the Lorentz form in this basis.
+We saw above that when we represent constructions as linear maps $A \colon \R^n \to V$, many constraints can be expressed by fixing entries of the Gram matrix $A^\dagger A$, which can be written more conveniently as $A^\top Q A$ when we choose an identification $V \cong \R^5$. This is an example of a *low-rank factorization* problem. In this section, we'll express the problem in a way that's useful for numerical computations.
+
+We'll find a non-negative function $f$ on the space of linear maps $\mathbb{R}^n \to V$ which vanishes at the matrices representing constructions that satisfy the constraints. Finding a global minimum of the *loss function* $f$ will be equivalent to finding a construction that satisfies the constraints. The loss function will be a quadratic polynomial in matrix entries, and its first and second derivatives will be straightforward to compute using a typical linear algebra package. This makes it easy to search for minima using Newton's method with backtracking.
+
+#### The Frobenius product
+
+For any dimensions $n$ and $m$, the inner products on $\mathbb{R}^n$ and $\mathbb{R}^m$ induce an inner product $\langle\!\langle X, Y \rangle\!\rangle = \operatorname{tr}(X^\top Y)$ on the space of matrices $\mathbb{R}^n \to \mathbb{R}^m$. We'll call this inner product the *Frobenius product*, because the associated norm, defined by $\|X\|^2 = \langle\!\langle X, X \rangle\!\rangle$, is called the *Frobenius norm*. The entry-wise dot product of $X$ and $Y$ gives $\langle\!\langle X, Y \rangle\!\rangle$, so the sum of the squares of the entries of $X$ gives $\|X\|^2$. More generally, in any orthonormal basis $v_1, \ldots, v_n$ for $\mathbb{R}^n$ we have
+\[ \langle\!\langle X, Y \rangle\!\rangle = \sum_{k \in [n]} \langle X v_k, Y v_k \rangle. \]
+
+#### The loss function
+
+Consider the matrices $\mathbb{R}^n \to \mathbb{R}^n$ whose entries vanish at the indices where the Gram matrix is unconstrained. They form a subspace $C$ of the matrix space $\operatorname{End}(\mathbb{R}^n)$. Let $\mathcal{P} \colon \operatorname{End}(\mathbb{R}^n) \to C$ be the orthogonal projection with respect to the Frobenius product. The constrained entries of the Gram matrix can be expressed uniquely as a matrix $G \in C$, and the linear maps $A \colon \mathbb{R}^n \to V$ that satisfy the constraints form the zero set of the non-negative function
+\[ f(A) = \|G - \mathcal{P}(A^\top Q A)\|^2\]
+on $\operatorname{Hom}(\mathbb{R}^n, V)$. Finding a global minimum of the *loss function* $f$ is thus equivalent to finding a construction that satisfies the constraints.
+
+#### The first derivative of the loss function
+
+*Writeup progress. Implemented in `engine-proto/gram-test/Engine.jl` on the `gram` branch.*
+
+#### The second derivative of the loss function
+
+*Writeup progress. Implemented in `engine-proto/gram-test/Engine.jl` on the `gram` branch.*
+
+#### Finding minima
+
+*Writeup progress. Implemented in `engine-proto/gram-test/Engine.jl` on the `gram` branch.*
+
+### Reconstructing a rigid subassembly
+
+Suppose we can find a set of vectors $\{a_k\}_{k \in K}$ whose Lorentz products are all known. Restricting the Gram matrix to $\mathbb{R}^K$ and projecting its output orthogonally onto $\mathbb{R}^K$ gives a submatrix $G_K \colon \mathbb{R}^K \to \mathbb{R}^K$ whose entries are all known. Suppose in addition that the set $\{a_k\}_{k \in K}$ spans $V$, implying that $G_K$ has rank five.
+
+Write $G_K$ as $L_K U_K$, where the matrix $U_K$ is a row echelon form of $G_K$, and the the matrix $L_K$ encodes the sequence of row operations that restores $G_K$. Since $G_K$ has rank five, the first five rows of $U_K$ are non-zero, and the rest are zero. Thus, the first five columns of $L_K$ are non-zero, and the rest are zero. Since $G_K$ is symmetric, we should have $L_K = U_K^\top D_K$, where $D_K$ is a diagonal matrix that has one negative entry and four positive entries at the pivot positions on the diagonal.
+
+Let $A_K \colon \R^K \to V$ be the restriction of $A$. The first five rows of $U_K$ form the matrix of $A_K$ in some orthogonal basis for $V$. The matrix $D_K$ describes the Lorentz form in this basis. By looking at $D_K$, it should be possible to rewrite $U_K$ in our chosen basis for $V$.
 
 ### Consistency constraints on the Gram matrix
 
-Since $G$ has rank five, every six-by-six minor is zero. Some of the entries of $G$ are known, because they're specified by constraints. Among the unknown entries, we want to treat some as independent variables, and the rest as dependent variables. We choose the independent variables so that each dependent variable is part of a six-by-six minor in which all the other entries are known or independent. Knowing that every six-by-six minor is zero then gives us a quadratic equation for each dependent variable.
+Since $G$ has rank five, every six-by-six minor is zero. Some of the entries of $G$ are known, because they're specified by constraints. Let's treat some of the unknown entries as independent variables, and the rest as dependent variables. Whenever we find a six-by-six submatrix where one entry is dependent and the rest are known or independent, knowing that the corresponding minor is zero gives us a quadratic equation for the dependent variable. Treating that dependent variable as "1-knowable" may provide further six-by-six submatrices where one entry is dependent and the rest are known, independent, or 1-knowable. The resulting quadratic equation for the dependent variable makes that variable "2-knowable", and the process continues.
 
-For $G$ to be realizable as a Gram matrix, it's necessary for each dependent variable to be real. The discriminants of the equations for the dependent variables thus give us a system of consistency constraint inequalities. To solve these inequalities, we need to distinguish between three pretty different cases.
-
-1. The solution space has non-empty interior—or, equivalently, codimension zero.
-2. The solution space has empty interior—or, equivalently, positive codimension.
-3. The solution space is empty.
+For $G$ to be realizable as a Gram matrix, it's necessary for each dependent variable to be real. Whenever we have a quadratic equation for a dependent variable, the discriminant of the equation gives us a consistency constraint inequality. The complexity of the consistency constraint for an $n$-knowable variable gets more complicated as $n$ increases.
 
 ### Choosing dependent variables