Can I publish if I develop a tool, but it solve non important problem only? (Mathematics) My research is in mathematics.
I have developed an interesting quantity X. Combined with other quantity Y, I can solve a certain problem.
However,

I don't know if the problem itself is important or not.
It needs to combined with other tools to work.

Can I still publish my idea?
Based on your reply to @DanielHatton's comment, you may have some research to make in that subfield to know about uses of the tool you discovered. Make your research in the subfield your tool may apply to and talk to people.

If you find something: great, you now have an application for your tool
If you don't find anything: it doesn't mean your tool doesn't have any application, let me explain.

In my humble opinion, publishing findings which may seem to have no clear application at the moment could still be useful years down the line. There are examples in history where research work had no application for a long time before..it did (anyone having such an example in mind is welcome to edit).
(Unfortunately?) not all papers are ground breaking. Yours probably won't be (as the vast majority of papers), but it might, sooner or later.
So I would encourage you to write a paper about it. You also mentioned that you are a student and I think that practising paper writing is valuable for students. You'll have some experience in case you want to become an academic, and if you don't, papers always look good on a resume.