# What is (chebyshev-function) tag for?

When you look at questions currently tagged , you will see questions from various areas there. (Number theory, Chebyshev polynomials, Chebyshev inequality,...) The tag-info for this tag is currently empty.

My first guess would be that this tag is supposed to be used for questions about Chebyshev functions such as $\vartheta(x)$ and $\psi(x)$, which are used in analytic number theory. (But there might be also some other meanings of this term I am unaware of.)

I think that it would be good to:

• Agree what this tag should be used for.
• Fill in the tag-excerpt and tag-wiki, explaining when the tag should be used.
• Clean up this tag. (Remove the tag from the questions belong there. Add the tag to the questions about Chebyshev functions.)
• If incorrectly tagged questions will keep appearing even after the tag-wiki is added, we might also create a comment template explaining correct usage of this tag.

But I wanted to get some input from other community members first.

I searched a bit whether this tag has already been discussed here on meta. The only thing I found was a mention of this tag in this answer:

Cautionary tale: is full of questions about Chebyshev-whatever, but not -function. At a glance, I have not found a single question about actual Chebyshev functions there. It is not unusual for rarely-used tags with few followers to degenerate into such a mess.

EDIT: Since nobody objected to the idea that this tag - if we keep it - should only be for questions about Chebyshev functions in number theory, I went ahead and created tag-wiki and tag-exceprt.

I have also started retagging questions which do not belong here, see this list. They are mostly about Chebyshev polynomials and Chebyshev inequality. Since I am not very familiar with these two topics, some people with more experience in these areas might have a look at those questions and choose better tags, if needed.

Of course, if there is support for the suggestion to get rid of the tag entirely - as suggested in the AlexR's answer - this still can be done. (Although in that case we will have remove the tag-wiki, since the tags having tag-wiki are not removed from the system.)

• Those who really are about chebychef functions seem sufficiently tagged by analytic-number-theory, so maybe we should just soft-delete the tag. I didn't see any question wich would need retagging after removing chebychev-function. If we keep it I suggest notifying about chebychev-polynomials in the wiki since it seems one of the most common mistags. – AlexR Feb 10 '15 at 8:22
• @AlexR Maybe it would be reasonable to post your comment as an answer. (So that we see from the voting whether other users agree/disagree with the removal of the tag.) Thanks also for the suggestion about (chebyshev-polynomials), it sounds very reasonable. – Martin Sleziak Feb 10 '15 at 8:29
• Seeing some of the questions in this tag makes me wonder whether it would be reasonable to create also a tag for Chebyshev's inequality. (But this is not relevant for the discussion about (chebyshev-function) tag.) – Martin Sleziak Feb 10 '15 at 9:55
• Here is a list of questions currently tagged (chebyshev-function). – Martin Sleziak Feb 13 '15 at 19:08

Suggestion for the tag-info, in case we decide to keep the tag. (I made the post CW, feel free to make any reasonable improvements.)

Tag-excerpt:

For questions about Chebyshev functions $\vartheta(x)$ and $\psi(x)$, which are often used in number theory. For questions about Chebyshev polynomials, use the (chebyshev-polynomials) tag.

Tag-wiki:

This tag is intended for questions about Chebyshev functions $\vartheta(x)$ and $\psi(x)$, which are often used in number theory. These functions are defined as $$\vartheta(x)=\sum_{p\le x} \log p$$ and $$\psi(x) = \sum_{p^k\le x}\log p=\sum_{n \leq x} \Lambda(n) = \sum_{p\le x}\lfloor\log_p x\rfloor\log p.$$

For questions about Chebyshev polynomials, use the tag.

Those questions which really are about s seem sufficiently tagged by , so maybe we should just soft-delete the tag (remove, don't blacklist).

I didn't see any question which would need retagging after removing , but if we decide to keep it I suggest notifying about in the wiki since it seems one of the most common mistags.

• I disagree about removing the tag only because all those questions belong to analytic-number-theory. Compare with derivatives: it's not because we have calculus that we don't need derivatives (probably there are better examples). – punctured dusk Feb 10 '15 at 12:23
• @barto "seem sufficiently tagged by" is a stronger observation than "are a subset of". – AlexR Feb 10 '15 at 12:24
• I agree with @AlexR. Unlike calculus, analytic number theory is a small enough section of mathematics (in the sense of questions on math.stackexchange) that further subdividing is unnecessary. – Peter Humphries Feb 12 '15 at 18:53
• I went through questions tagged only chebyshev-functions. I have added some tag, but if you have idea for better tags, please, go ahead and retag them. – Martin Sleziak Feb 13 '15 at 7:19
• @MartinSleziak All three are not about the function, but are about the inequality, so the tag can actually be removed from them regardless of our procedure with the tag itself. – AlexR Feb 13 '15 at 12:11