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Feb 13 at 21:45 comment added Christian Remling @Keen-ameteur: I think I have a slightly clearer picture now, and it seems this is still not working, though there are similar in spirit statements that are true. See the latest edit please.
Feb 13 at 21:43 history edited Christian Remling CC BY-SA 4.0
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Feb 13 at 16:35 comment added Keen-ameteur I think I may have misstated the condition I wanted. I've edited the question. Under my original intention and my corrected edit, $q_n\leq n-1$ for your proposed $V_n$. I'm sorry for the confusion.
Feb 13 at 15:35 comment added Christian Remling @Keen-ameteur: Sorry, it's not completely clear to me what you are saying in your last comment, but I edited my answer and I hope this will address everything. Also, $PH_nP$ is not the zero operator in my example for a fixed projection, it's the (restricted) Laplacian.
Feb 13 at 15:35 history edited Christian Remling CC BY-SA 4.0
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Feb 13 at 15:19 comment added Keen-ameteur It is very likely that I am confused, but $H_n$ have to stabilize near the origin. Namely, if $H_n\to 0$ then $M_n\equiv 0$. I think that for all $n\geq n_0$, we have $P_{q_{n_0}}H_nP_{q_{n_0}} = M_{n_0}$. So if $H_n\to 0$ that must mean that $M_{n_0}\equiv 0$, doesn't it?
Feb 13 at 15:06 comment added Christian Remling @Keen-ameteur: This doesn't change anything essentially. I only need to take $q_n$ sufficiently large and will then still have an eigenvalue $\simeq E$. Admittedly, it will have moved a bit, but we then have $\dim\chi_{(E-\epsilon,E+\epsilon)}(M_n)\ge 1$, $(E-\epsilon,E+\epsilon)\cap\sigma (\Delta)=\emptyset$.
Feb 13 at 14:58 comment added Keen-ameteur Thanks for your answer. I was asking that $H_n$ projected onto the finite box, what I denoted by $M_n$, have $E$ in its spectrum with multiplicity bounded from below. It seems like in the example you wrote, $\dim \chi_{\{ E\}}(M_n)= 0$ eventually.
Feb 13 at 14:32 history answered Christian Remling CC BY-SA 4.0