We consider the effect of Gaussian perturbations on least-squares residuals, orthogonal projections, and QR-type algorithms. The problem that motivated our investigations is as follows: suppose that a full column-rank matrix \(B\in\mathbb{R}^{m\times n}\) has already been computed, and suppose that a new normalized column \(q=(x+y)/\|x+y\|_2\) is to be appended to \(B\), where \(x\perp\operatorname{span}(B)\) is the ideal orthogonal component and \(y\) represents the orthogonalization error. How large can the condition number \(κ([B,q])\) of the resulting matrix \([B,q]\) become? While we provide a Weyl-type bound on the singular values of \([B,q]\), in terms of the extremal singular values of \(B\) and the quantity \(\|B^T y\|_2/\|x+y\|_2\), we also derive exact probability laws for norms and projection residuals under Gaussian perturbations. Finally, we use these probability laws to derive probabilistic condition-number bounds for QR-type processes with imperfect orthogonalization and exact normalization.

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