diff --git a/Gram-matrix-parameterization.md b/Gram-matrix-parameterization.md index c3402aa..e3785e5 100644 --- a/Gram-matrix-parameterization.md +++ b/Gram-matrix-parameterization.md @@ -2,28 +2,13 @@ ### Construction elements as vectors -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 $\mathbb{R}^{1,4}$. If we normalize these vectors to pseudo-length $\pm 1$, and choose a lightlike vector on the 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 -#### Introducing the Gram matrix - -The vectors $a_1, \ldots, a_n \in \mathbb{R}^{1,4}$ representing the elements of our construction can be encoded in a linear map $A \colon \mathbb{R}^n \to \mathbb{R}^{1,4}$, whose matrix is +The vectors $a_1, \ldots, a_n \in V$ representing the elements of our construction can be encoded in a linear map $A \colon \mathbb{R}^n \to V$, whose matrix is \[ \left[\begin{array}{cccc} \rule{0.5pt}{16pt} & \rule{0.5pt}{16pt} & & \rule{0.5pt}{16pt} \\ a_1 & a_2 & \cdots & a_n \\ \rule{0.5pt}{16pt} & \rule{0.5pt}{16pt} & & \rule{0.5pt}{16pt} \end{array}\right] \] -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 $(\_\!\_, \_\!\_)$ on $\mathbb{R}^{1,4}$. - -#### The geometry of the Gram matrix +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 image of $A$. The symmetric bilinear form $\langle\_\!\_, G\_\!\_\rangle$ is the pullback of the Lorentz form along $A$: -\[\begin{align*} \langle\_\!\_, G\_\!\_\rangle & = \langle\_\!\_, A^\top A\_\!\_\rangle \\ & = (A\_\!\_, A\_\!\_). \end{align*}\] -To confirm that $G$ is the Gram matrix of $a_1, \ldots, a_n$, observe that -\[\begin{align*} G_{jk} & = \langle e_j, G e_k \rangle \\ & = (A e_j, A e_k) \\ & = (a_j, a_k), \end{align*}\] -where $e_1, \ldots, e_n$ is the standard basis for $\mathbb{R}^n$. - -#### The rank of the Gram matrix - -Since inner products and Lorentz forms are both non-degenerate, the kernel of $\langle\_\!\_, G\_\!\_\rangle$ is $\ker A$. The form $\langle\_\!\_, G\_\!\_\rangle$ on $(\ker A)^\top$ is isometric, through $A$, to the Lorentz form on $\operatorname{im} A$. It follows that if $A$ is onto, then $\langle\_\!\_, G\_\!\_\rangle$ has signature - -$-$ | $+$ | $\cdot$ ----|---|--- -$1$ | $4$ | $n-5$ \ No newline at end of file +\[\begin{align*} \langle\_\!\_, G\_\!\_\rangle & = \langle\_\!\_, A^\top A\_\!\_\rangle \\ & = (A\_\!\_, A\_\!\_). \end{align*}\] \ No newline at end of file