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Suppose I'm solving $Ax=b$ using row-action method like Kaczmarz for $m\times n$ matrix A with $m\approx \infty$ and have $H_k=\frac{1}{k}A_k^T A_k$ which is an estimate of the Hessian obtained from random $k$ rows of $A$.

How should I use $H_k$?

The problem of diving by $H_k$ is that it appears to underestimate the variance of $H_\infty$ in directions of bottom eigenvectors of $H_k$. Realistically, only first $K-1$ directions may have an accurate estimate and normalizing in direction $K$ is numerically unstable. Natural approach is precondition using $H_k+\epsilon I$ or by using first $K$ factors of truncated SVD of $H_k$. How to choose $\epsilon$ or $K$?

Here's an experiment I did, for $n=784$ and $k=60000$, I can estimate $H_k$ using $60,000$ random rows, and evaluate it on a different set of $10,000$ rows. Eyeballing the graph shows that blow-up starts to happen around $K=600$.

Additionally, I tried with $k=1000$ and similarly found that I only first $K=320$ directions have an accurate estimate of variance.

Is there a more rigorous way of obtaining these cutoffs?

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  • $\begingroup$ I can't speak to this specific application, but the typical way of choosing $K$ for the truncated SVD is to pick some tolerance $\eta$ and let $K$ be the largest integer such that $$\frac{\sum_{i=K}^n \sigma_i^2}{\sum_{i=1}^n \sigma_i^2} < \eta$$ is true. This can be rewritten in many ways but it boils down to "Choose the smallest $K$ that represents the $1-\eta$ fraction of the variance. $\endgroup$
    – whpowell96
    Nov 20, 2023 at 19:17
  • $\begingroup$ Given $\eta$ I can choose $K$, then question is then -- how to pick $\eta$? Because $H_k$ is estimated from $k$ samples, $\eta$ should depend on $k$ somehow $\endgroup$ Nov 20, 2023 at 19:23
  • $\begingroup$ In my experience, people either just choose $\eta$ small, e.g., $10^{-2}$, or they plot the singular values of the matrix and look for where a large drop in the magnitude of the singular values occurs and truncate there. $\endgroup$
    – whpowell96
    Nov 20, 2023 at 19:26

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