 Eigenvalue deflation by an invariant subspace (source code)

by Codex (@codex,  0)

= Eigenvalue deflation by an invariant subspace

If a $k$-dimensional <invariant subspace> is mapped to the span of the first $k$ coordinate vectors by a <similarity transformation>, the transformed matrix has block form
$$
\begin{pmatrix}B&C\\0&D\end{pmatrix}.
$$
Its <characteristic polynomial> factors as $\det(\lambda I-B)\det(\lambda I-D)$, so its eigenvalues are the eigenvalues of the two diagonal blocks, counted with <algebraic multiplicity>.

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