diff --git a/Gram-matrix-parameterization.md b/Gram-matrix-parameterization.md index e4c4752..e6c4927 100644 --- a/Gram-matrix-parameterization.md +++ b/Gram-matrix-parameterization.md @@ -41,21 +41,11 @@ on $\operatorname{Hom}(\mathbb{R}^n, V)$. Finding a global minimum of the *loss Write the loss function as \[ \begin{align*} f & = \|\Delta\|^2 \\ & = \operatorname{tr}(\Delta^\top \Delta), \end{align*} \] where $\Delta = G - \mathcal{P}(A^\top Q A)$. Differentiate both sides and simplify the result using the transpose-invariance of the trace: -\[ -\begin{align*} -df & = \operatorname{tr}(d\Delta^\top \Delta) + \operatorname{tr}(\Delta^\top d\Delta) \\ -& = 2\operatorname{tr}(\Delta^\top d\Delta). -\end{align*} -\] +\[ \begin{align*} df & = \operatorname{tr}(d\Delta^\top \Delta) + \operatorname{tr}(\Delta^\top d\Delta) \\ & = 2\operatorname{tr}(\Delta^\top d\Delta). \end{align*} \] To compute $d\Delta$, it will be helpful to write the projection operator $\mathcal{P}$ more explicitly. Let $\mathcal{C}$ be the set of indices where the Gram matrix is unconstrained. We can express $C$ as the span of the standard basis matrices $\{E_{ij}\}_{(i, j) \in \mathcal{C}}$. Observing that $E_{ij} X^\top E_{ij} = X_{ij} E_{ij}$ for any matrix $X$, we can do orthogonal projection onto $C$ using standard basis matrices: \[ \mathcal{P}(X) = \sum_{(i, j) \in \mathcal{C}} E_{ij} X^\top E_{ij}. \] It follows that -\[ -\begin{align*} -d\mathcal{P}(X) & = \sum_{(i, j) \in \mathcal{C}} E_{ij}\,dX^\top E_{ij} \\ -& = \mathcal{P}(dX). -\end{align*} -\] +\[ \begin{align*} d\mathcal{P}(X) & = \sum_{(i, j) \in \mathcal{C}} E_{ij}\,dX^\top E_{ij} \\ & = \mathcal{P}(dX). \end{align*} \] Since the subspace $C$ is transpose-invariant, we also have \[ \mathcal{P}(X^\top) = \mathcal{P}(X)^\top. \] We can now see that @@ -66,24 +56,9 @@ d\Delta & = -\mathcal{P}(dA^\top Q A + A^\top Q\,dA) \\ \end{align*} \] Plugging this into our formula for $df$, and recalling that $\Delta$ is symmetric, we get -\[ -\begin{align*} -df & = 2\operatorname{tr}(-\Delta^\top \big[\mathcal{P}(A^\top Q\,dA)^\top + \mathcal{P}(A^\top Q\,dA)\big]) \\ -& = -2\operatorname{tr}\left(\Delta\,\mathcal{P}(A^\top Q\,dA)^\top + \Delta^\top \mathcal{P}(A^\top Q\,dA)\big]\right) \\ -& = -2\left[\operatorname{tr}\big(\Delta\,\mathcal{P}(A^\top Q\,dA)^\top\big) + \operatorname{tr}\big(\Delta^\top \mathcal{P}(A^\top Q\,dA)\big)\right] \\ -& = -4 \operatorname{tr}\big(\Delta^\top \mathcal{P}(A^\top Q\,dA)\big), -\end{align*} -\] +\[ \begin{align*} df & = 2\operatorname{tr}(-\Delta^\top \big[\mathcal{P}(A^\top Q\,dA)^\top + \mathcal{P}(A^\top Q\,dA)\big]) \\ & = -2\operatorname{tr}\left(\Delta\,\mathcal{P}(A^\top Q\,dA)^\top + \Delta^\top \mathcal{P}(A^\top Q\,dA)\big]\right) \\ & = -2\left[\operatorname{tr}\big(\Delta\,\mathcal{P}(A^\top Q\,dA)^\top\big) + \operatorname{tr}\big(\Delta^\top \mathcal{P}(A^\top Q\,dA)\big)\right] \\ & = -4 \operatorname{tr}\big(\Delta^\top \mathcal{P}(A^\top Q\,dA)\big), \end{align*} \] using the transpose-invariance and cyclic property of the trace in the final step. Writing the projection in terms of standard basis matrices, we learn that -\[ -\begin{align*} -df & = -4 \operatorname{tr}\left(\Delta^\top \left[ \sum_{(i, j) \in \mathcal{C}} E_{ij} (A^\top Q\,dA)^\top E_{ij} \right] \right) \\ -& = -4 \operatorname{tr}\left(\sum_{(i, j) \in \mathcal{C}} \Delta^\top E_{ij}\,dA^\top Q A E_{ij}\right) \\ -& = -4 \operatorname{tr}\left(dA^\top Q A \left[ \sum_{(i, j) \in \mathcal{C}} E_{ij} \Delta^\top E_{ij} \right]\right) \\ -& = -4 \operatorname{tr}\big(dA^\top Q A\,\mathcal{P}(\Delta)\big) \\ -& = \langle\!\langle dA,\,-4 Q A\,\mathcal{P}(\Delta) \rangle\!\rangle. -\end{align*} -\] +\[ \begin{align*} df & = -4 \operatorname{tr}\left(\Delta^\top \left[ \sum_{(i, j) \in \mathcal{C}} E_{ij} (A^\top Q\,dA)^\top E_{ij} \right] \right) \\ & = -4 \operatorname{tr}\left(\sum_{(i, j) \in \mathcal{C}} \Delta^\top E_{ij}\,dA^\top Q A E_{ij}\right) \\ & = -4 \operatorname{tr}\left(dA^\top Q A \left[ \sum_{(i, j) \in \mathcal{C}} E_{ij} \Delta^\top E_{ij} \right]\right) \\ & = -4 \operatorname{tr}\big(dA^\top Q A\,\mathcal{P}(\Delta)\big) \\ & = \langle\!\langle dA,\,-4 Q A\,\mathcal{P}(\Delta) \rangle\!\rangle. \end{align*} \] From here, we get a nice matrix expression for the negative gradient of the loss function: \[ -\operatorname{grad}(f) = 4 Q A\,\mathcal{P}(\Delta). \] This matrix is stored as `neg_grad` in the Rust and Julia implementations of the realization routine. @@ -93,12 +68,7 @@ This matrix is stored as `neg_grad` in the Rust and Julia implementations of the Recalling that \[ -d\Delta = \mathcal{P}(dA^\top Q A + A^\top Q\,dA), \] we can express the derivative of $\operatorname{grad}(f)$ as -\[ -\begin{align*} -d\operatorname{grad}(f) & = -4 Q\,dA\,\mathcal{P}(\Delta) - 4 Q A\,\mathcal{P}(d\Delta) \\ -& = 4 Q\big[{-dA}\,\mathcal{P}(\Delta) + A\,\mathcal{P}(-d\Delta)\big]. -\end{align*} -\] +\[ \begin{align*} d\operatorname{grad}(f) & = -4 Q\,dA\,\mathcal{P}(\Delta) - 4 Q A\,\mathcal{P}(d\Delta) \\ & = 4 Q\big[{-dA}\,\mathcal{P}(\Delta) + A\,\mathcal{P}(-d\Delta)\big]. \end{align*} \] In the Rust and Julia implementations of the realization routine, we express $d\operatorname{grad}(f)$ as a matrix in the standard basis for $\operatorname{End}(\mathbb{R}^n)$. We apply the cotangent vector $d\operatorname{grad}(f)$ to each standard basis matrix $E_{ij}$ by setting the value of the matrix-valued 1-form $dA$ to $E_{ij}$. #### Finding minima