"combinatorial number theory"
"linear fractional transformations"
The software forbids these two tags because they are too long. Should the length limits be adjusted? Or is there some way of making special exceptions?
Should the length limits be adjusted?
For the most part 25 characters suffice, and it is also a road-block against exaggeration in tag names. Say, I would not want to see tags like limits-at-infinity-without-lhospital-involving-trigonometric-functions.
However, it is a fact, and you gave some example, that there are some legitimate and natural tag names that are longer than 25 characters. Thus, I think at least something more flexible than a hard limit would be desirable.
Or is there some way of making special exceptions?
As far as I know, no. On some sites they came up with the workaround of chopping up the tag name and then creating synonyms to piece things together. But, this is not a real solution either.
Thus, for the cases you mention, I propose to use abbreviations, like contracting the 'theory.'
Here is an old and somewhat popular feature request on the main Meta site