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I have a large $m\times n$ data matrix $A$, $m>n$, and response $m$-vector $b$. I need to calculate $E = ||Ax-b||_2$ as quickly as possible, where $x$ is the least squares solution. I don't need to actually know $x$. The data matrix $A$ may not have full rank. What is the fastest algorithm to calculate $E$ in this case?

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