On Reference for invariance of essential spectrum under relatively compact perturbations, Michael Renardy voted to close with the comment "This site should not become a shortcut to substitute for literature searches", and it seems at least two others are of the same opinion. Respectfully, I'm not sure I agree, either in the context of this question or in general, and I thought I would solicit the community's input. Indeed, I thought this was the whole point of the reference-request tag, and a major part of what makes this site an asset to the community.
Searching the literature for a specific result is a tedious and often unrewarding task, especially for a junior researcher or someone looking for results outside their own area of expertise who may not even know where to begin. Online search tools are not very good at this: the desired paper may state the result in different language, or prove a generalization. The situation gets much worse if one has to search for symbols.
It's a time-honored tradition to shortcut this drudgery by asking an expert colleague whom you suspect might know immediately where to find your result, or at least where to start looking. But not everyone has an expert colleague down the hall, which to my mind is exactly the reason why MO was started.
There's a limit, of course: we want questions which are "research level", which by consensus seems to be roughly defined as "beyond a standard first-year graduate course". I think the question at hand certainly meets that standard: I have a Ph.D. and what I like to think is a working knowledge of basic functional analysis, and I don't have any idea where to find the requested result.
I don't see such questions as making a disrespectful demand on the time of others, either; a reference request can be answered in seconds by someone who knows where to find what's wanted, and everyone else can shrug and move on. Many people here invest much more time in writing detailed answers to complicated questions. I personally would gladly take the time to answer this question if I knew an appropriate reference, and would do the same for any other reference request within my limited expertise.
I'd appreciate hearing any other thoughts.