Implicit function theorem Wikpedia example I refer to the implicit function theorem as stated on Wikipedia (https://en.wikipedia.org/wiki/Implicit_function_theorem)

I don't understand the example given later (unit circle)

Q1) What do they mean by "the Y in the statement of the theorem is just the number 2b"? I don't see a captial Y in the statement of the theorem.
Q2) Since the matrix is 1x2 (non square), isn't it non-invertible hence the theorem does not apply here? Quite confused about this.
Thanks for any help.

The $Y$ refers (on the wiki page) to the matrix written a few lines above the statement of the theorem (the editing of the wiki page could be better).
It is the partial derivatives wrt the y's.
This very same place also shows that the (rectangular) matrix should be interpreted as $[X|Y]$ where $X=2a$ and $Y=2b$ ($Y$ is a 1 by 1 matrix, thus invertible when $b\neq 0$).
