@WillieWong Do you think it would be reasonable to split your question into two questions - one of them about estimation-theory and estimation, the second one about approximation-theory and approximation.

It seems that the answers so far (with exception of GEdgar's answer) address one of these two.

But probably it's not necessary, I do not expect many more answers appear in that thread.

@MartinSleziak Since I already asked it that way, and I doubt there will be more informative answers, I guess we can leave it like that. But you are right: my ignorance made me think that the two maybe related, when they are in fact not.

@WillieWong Thanks for the reply. And I definitely would not call it ignorance. My first impression was similar to yours - that everything will probably be merged under same tag.

I am not sure about that anymore, not even in the case of (approximation), as I explained in my comment.

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BTW WillieWong I see you're dealing with new tags, too; you have removed high-school which was introduced here.

From the newly introducted tags, group-actions seems to be reasonable (in my opinion).

Ok, now as I see your recent edit, you probably don't think that group-actions should have their own tag.

BTW at MO there is group-actions tag.

@MartinSleziak you are right. I read that question too quickly. I thought it was about the group operation of the topology group itself, but on second reading it was about G's action on another topological space. So I just reverted the edit. (The tag itself is reasonable; I just mistakenly thought it was misapplied.)

ok, as I wrote before, I think that this tag is reasonable, too.

@MartinSleziak "high school" on the other hand, I cannot see any point to it.

Since we already had plenty of questions about group actions, I have no doubt that the tag will be populated by new questions.

@WillieWong I wasn't sure about the purpose of that tag.

I saw that J.M. made a revision to that question, so I did nothing about the tag. (I thought he saw the tag and decided to keep it.) But maybe he did not notice that tag.

Or maybe he wasn't sure whether to keep or remove - just as me.

@MartinSleziak A long time ago we had a high school tag for tagging questions of "high school level" , it led to some fiery discussions on meta about why such a tag is bad. (Discouraging users etc.) In this case the tag is used differently, seemingly to indicate the rather arbitrary distinction of an age group (14 - 18?). I can't see why such a tag would be useful.

It was probably before my time here.

Or at least before I started to care a little more about tagging.

@MartinSleziak J.M. may have added big list because the question was flagged for CW.

(I meant, for the same reason that users have flagged the question for CW.)

I see the discussion about that tag in meta: Can we please delete the high-school tag?.

I think the OP just tagged high-school because there aren't really any good tags for the question. And I think that there aren't any good tags because it is not a good question for this site. (Just my opinion.)

I am kind of inclusionist, both as far as questions and tags are concerned.

As can be seen from my promoting of tags such as filters, limsup or compactness.

So my opinion would be to keep the question - but it's not relevant for this chatroom.

Anyway, thanks for both mentioning the older discussion about high-school and putting group-actions back.