I noticed that Eigen has an undocumented function implementing Higham's 1988 paper on 1-norm condition number estimation. Would you be interested in adding Higham and Tisseur's algorithm (http://eprints.ma.man.ac.uk/321/1/35608.pdf) into Eigen as well? I have a draft of this functionality and a use case, so if you are interested I'm interested in participating in your project.
I personally have no need for that, but I don't oppose adding that.
If this is added to the public API, the existing condition number estimator should be exposed to the public API as well (and documented, and tested).
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