- #1
twoflower
- 368
- 0
Hi, I have some troubles understanding the basic facts about investigating the continuity of two-variable functions.
Our professor gave us very simple example to show us the basic facts:Very important is that projections are continuous, it means
[tex]
\pi_1 :[x,y] \longmapsto x
[/tex]
[tex]
\pi_2 :[x,y] \longmapsto y
[/tex]
are continuous.
Now let's have this function:
[tex]
f(x,y) = \sqrt{x^2+y^2}
[/tex]
It's continuous on the whole [itex]\mathbb{R}^2[/itex]. Why?
1. Step:
Projections are continuous
2. Step:
[tex]
x,y \longmapsto x^2 + y^2[/tex]...continuous
3. Step:
Square root is continuous on [itex][0,\infty)[/itex]
What I don't fully understand is the second step. I can't see why, from the fact that projections are continuous, we can say that
[tex]
x^2 + y^2
[/tex]
is continous. Of course I didn't expect any other result, when you look at it it's obvious that it will be continuous, but I just can't see the rigorous mathematical background.
You know, I see it is some equivalent of the limit of product of single variable function (? is it the right expression ?), but I don't think it's sufficient. I would need some equivalent for two-variable functions...
Hope you understand my problem :)
Thank you very much.
Our professor gave us very simple example to show us the basic facts:Very important is that projections are continuous, it means
[tex]
\pi_1 :[x,y] \longmapsto x
[/tex]
[tex]
\pi_2 :[x,y] \longmapsto y
[/tex]
are continuous.
Now let's have this function:
[tex]
f(x,y) = \sqrt{x^2+y^2}
[/tex]
It's continuous on the whole [itex]\mathbb{R}^2[/itex]. Why?
1. Step:
Projections are continuous
2. Step:
[tex]
x,y \longmapsto x^2 + y^2[/tex]...continuous
3. Step:
Square root is continuous on [itex][0,\infty)[/itex]
What I don't fully understand is the second step. I can't see why, from the fact that projections are continuous, we can say that
[tex]
x^2 + y^2
[/tex]
is continous. Of course I didn't expect any other result, when you look at it it's obvious that it will be continuous, but I just can't see the rigorous mathematical background.
You know, I see it is some equivalent of the limit of product of single variable function (? is it the right expression ?), but I don't think it's sufficient. I would need some equivalent for two-variable functions...
Hope you understand my problem :)
Thank you very much.
Last edited: