I am currently writing a paper that I wish to publish in a mathematics journal. During the course of my research, I have discovered a result that is aesthetically pleasing, i.e., contains a form of symmetry in its definition that can be seen by some as "elegant", and moreover connects several distant theorems together. However, this result turns out to be useless for practical use, furthermore it adds nothing to the other proofs and theorems presented in the paper. Is it worth publishing/mentioning, even as a corollary?
Yes, this is fine. Math papers very often contain results just because they are interesting or instructive, even if they do not seem to be "useful".
You could mention this when introducing the result, with something like "The following theorem may help to illustrate the connection between blah blah blah..."
Authors also sometimes signal this sort of thing by describing a result as "pleasant", "amusing", etc, though "elegant" is probably a little too egotistical.
If the referee feels it's too much of a digression, they might suggest you take it out. But I don't think this would be the difference between acceptance and rejection.
To add to Nate Eldredge's correct (and useful!) answer and to Alexander Woo's sarcastic quip highlighting the same point, one should keep in mind that pure mathematics is, by its very definition, the part of mathematics that seeks to study mathematical structures for the sake of the pure intellectual and aesthetic value of the mathematical ideas one is trying to discover. Yes, it helps that a lot of pure mathematics has turned out to be useful beyond the wildest dreams of the people who discovered it -- a totally weird phenomenon that no one seems to understand -- but that is not the primary concern (or even the secondary or tertiary concern, usually) of the pure mathematician.
Lack of (caring about) usefulness is a feature, not a bug.
Absolutely. Not just because, to many, the point of science and mathematics is understanding and appreciating the beauty of reality, but also because it might become practical in the future! I doubt the people who worked on number theory foresaw crytography, for instance, or the esoteric probability theory making its way into machine learning now.
Also, I think tying together distant theorems is a practical application in some sense. Or at least an educational one for practitioners who might be trying to understand something, and realize something useful based on your theorem tying it to something else.
As the other answer says, I think it makes sense as long as you make it coherent with the rest of the paper!
Engineer here. I marvel at all those math papers that contain nothing but "hey this looks cool!" I really like those. Some of these are even easy enough for me to understand :-) And do not fret about applications. You are doing math. You are doing theory. It's the engineers job to find an application for it.
(pats OP on the back)
Congratulations, you're now officially a Mathematician! Publish away.
On a slightly more serious note: Spend time working on a good introduction that communicates the pleasing elegance of your results (or rather the lack of pleasing elegance without them). If for some reason the journals/conferences think it's totally useless, they'll reject.