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Jan 17, 2022 at 18:52 comment added Fei Cao @ChristianRemling Yes I really want a essentially different argument than the argument based on Cauchy's interlacing theorem, if your approach more or less used the interlacing result that is probably not the approach I want to see...
Jan 17, 2022 at 18:41 comment added Christian Remling @ShannonStarr: Of course, in another sense you cannot escape the interlacing principle because the statement the OP wants proved is essentially a rephrasing of it.
Jan 17, 2022 at 18:36 comment added Christian Remling @FeiCao: If $V,W$ are subspaces with $\dim V>\dim W+k$, then $\dim V\ominus W>k$.
Jan 17, 2022 at 18:34 comment added Christian Remling @ShannonStarr: I just said "min-max" for ease of reference. What I need here is that if the quadratic form is $\le x$ on a $d$-dimensional subspace, then there are at least $d$ eigenvalues in $(-\infty,x]$. This is a very easy exercise.
Jan 17, 2022 at 15:48 comment added Shannon Starr The min-max principle is how you prove the interlacing theorem.
Jan 17, 2022 at 1:36 comment added Fei Cao May I know why "$B$ had $>m+k$ eigenvalues there, then we could find $>k$ orthogonal vectors $R(E_B(-\infty,x])$ (with $E$ denoting the spectral projection) that are also orthogonal to (the $m$-dimensional space) $R(E_A(-\infty, x])$"? It will be better if you can explain your notations in more detail as well.
Jan 16, 2022 at 23:30 history undeleted Christian Remling
Jan 16, 2022 at 23:28 history deleted Christian Remling via Vote
Jan 16, 2022 at 23:26 history answered Christian Remling CC BY-SA 4.0