8:44 AM
The fact that is shown among new tags made me wonder how it got there.
This is the only question with this tag:
1

My question is the following: if I know $f''(x)<0,~\forall~0\leq x\leq a$ and I know that $f'(c)=0$ where $c\in[0,a]$ and I also know $f(0)=0$, then is that true $f(x)>0,~\forall~x\in(0,c]$? I think it is correct.

I see in Google Cache that is was tagged .
Has the tag been removed recently? Isn't it more usual on MO that tags are replaced by rather than (as mentioned here and here)?

2 hours later…
11:01 AM
I tried this SEDE query. It found only this question as a question having the tag concavity.
So it seems quite plausible that the tag was removed by the tag-pruning process, which removes single occurrence tags. (I have recently asked on another meta: Is it possible to find out whether a tag has been removed by an automated process?)