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Let $K$ be a field, $A\in K^{n\times m}$ and $\lVert \cdot \rVert$ the Euclidean norm. Consider the problem: Find a $v\in K^m$ such that \begin{align} \lVert Av\rVert=\min_{\lVert x\rVert=1}\lVert Ax\rVert \end{align} This minimization problem can be solved for $K=\mathbb{R},K=\mathbb{C}$ with the SVD-algorithm. The corrosponding right-singular vector of the smallest singular value is a solution (If $A=U\Sigma V^*$ is the SVD and the singular values on the diagonal of $\Sigma$ are sorted by their absolute value then the rightmost column of $V$ solves the problem).

However, I am now looking for a more general method of solving this problem for any field $K$. Is there a generalization of the SVD-algorithm that maybe preserves this property?

Edit: The same method works for algebraic numbers (The singular value problem can be reduced to the eigenvalue problem which is closed under the algebraic numbers). The comments pointed quaternionics as well.

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  • $\begingroup$ The SVD works well with quaternionic matrices. $\endgroup$ Commented Feb 5, 2024 at 13:35
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    $\begingroup$ It won't work for "any" field $K$ - you at least need that the minimum value and a minimiser to be expressible in this field. $\endgroup$ Commented Feb 5, 2024 at 14:51
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    $\begingroup$ And what is a norm on a vector space over $K$? $\endgroup$ Commented Feb 5, 2024 at 14:54
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    $\begingroup$ Also, even over $K = \mathbb{R}$ or $K = \mathbb{C}$, the minimum in question can only be computed via the SVD if the norm being used on $K^m$ is the Euclidean norm (i.e., $2$-norm). For many other norms (e.g., the $p$-norms when $p \in (1,2) \cup (2,\infty)$), this problem is NP-hard. $\endgroup$ Commented Feb 5, 2024 at 15:55
  • $\begingroup$ The norm does not matter, but I'll write Euclidean norm in the question to make it easier to understand. $\endgroup$ Commented Feb 6, 2024 at 12:50

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