In the mathematical sciences, how much advisor contribution is expected on joint papers? I'm a PhD student in the math sciences.  I've written a few papers, each officially coauthored with my advisor, although essentially all of the research and writing was my own.  The story is the same for my advisors' other students; they regularly coauthor papers with my advisor on which all the work was theirs.

We are all unsure how we should feel about this.  On one hand, we regularly hear stories from other students about their advisor chipping in on projects, or encouraging them to write solo if unable, and we would all appreciate this sort of freedom, guidance, and help.  On the other hand, we all recognize that we are paid out of our advisor's grants, and we don't want to appear ungrateful or unprofessional.

I guess my question is: are we being taken advantage of, or is this a fairly standard feature of advisor/advisee relationships that we should just accept?

We've all tried to casually broach the topic of when coauthorship is expected or when solo authorship is okay, but we've mostly been met by blank stares and awkward subject changes.
I disagree with the given answer by Buzz in a few ways, especially since it does not address one decisive part of the question:


  We've all tried to casually broach the topic of when coauthorship is expected or when solo authorship is okay, but we've mostly been met by blank stares and awkward subject changes.


If there is a justification for the advisor being included as an author, then why has it not been brought up by said advisor when being confronted by his students? It would perhaps be understandable if the author just didn't pick up on the subtle clues his students were giving him, but that does not seem to be the case as per the indications of 'awkward subject changes' and 'blank stares'. It seems that the advisor is simply trying to avoid talking about the issue. Why would he do that? It certainly indicates that something fishy may be going on here. 

Then, the other answer asserts that nobody is less impressed by a student who always has her advisor as a co-author*. This is verifiably wrong, which you can prove, being a mathematician, by "contradiction": if nobody is less impressed by a student who publishes papers of a certain quality with her advisor, then a direct implication is that nobody is impressed more by a student who publishes papers of an equivalent quality independently of her advisor. This is wrong. It shows great promise to be able to do independent research as a young student. 

But I hesitate to even make that point. This should not be about recognition but about a matter of principle: if your advisor did not contribute to the research (either through substantial amounts of guidance or by actively doing it) or the writing, your advisor is not an author of the resulting paper, and making him one is a direct lie. 

In your situation, judging by your post, it seems clear that the students are doing all of the work (in fact, that's a direct quote from your post). If true, you should be worried, both by the ethical issues that exist if you continue this practice, and the potential aftermath (e.g, in regards to your relationship with your advisor) if you decide to take this further (which you should, but that's just an opinion). 

Note that all of this is entirely dependent on the assumption that what's going on here is not normal. If it is, if advisors are usually co-authors on all their student's papers despite no contribution, if that is understood by everyone, then you are not lying, you will get all the recognition, and there's really no issue at all ... but how are you supposed to know whether this is normal in your field or cultural domain? We cannot really answer that for you without a bit more information. But your advisor can ... and this brings us full circle: why is the advisor not willing to explain to you whether this is normal?

*.. I now realize that I may have misread the other answer. It's not quite clear whether by "work" they meant "effort" or "actual quality of the mathematical content". Clearly, I took it to mean the former.