How supposed I imagine the following equations?

[tomcenjerrym]

What is SUBSET on “open subset of R$$^{n}$$"?
How supposed I imagine the following equations?

u : U → R

 

[JonF]

it's the same as any other subset.

U is a subset of P if

for all x in U x is also in P.

So what does it mean to be an element of an open set of R^n?

u is probably a function.

It assigns each element in U to one element in R