How to ask someone to collaborate on a joint paper?
I'm not a mathematician, and in my field sole-authored papers are unusual, so my answer might not be entirely relevant, but for what it's worth:

You don't usually ask someone to collaborate on a paper because you are stuck. Either the collaboration arises naturally through a joint project, or you ask someone to join your paper because they have some specific expertise or experience that you think can add to what is already going to constitute a good, publishable paper. In the latter case, I have had success by sending an email briefly outlining the goals of the paper, its importance, and the timeline on which I intend to have the paper completed. I finish the email by stating what I think they could add to the paper and asking them if they are interested in a co-authorship on this basis. I might already have a draft of the paper ready, and if so, I will attach it so that they can see that I am serious and the paper has a good prospect of timely completion.

Also: a bit of flattery doesn't go astray. Refer to "your important work on..." or "your extensive experience in..." among the reasons for approaching them.

If in fact, you are not approaching someone to collaborate because of something specific that they can add to an already solid paper, but merely looking for someone to help with a work in progress, I suggest approaching someone at a junior level who already knows you, and asking for help.