# Continuity of fog and f/g on standard topology.

• Oct 16th 2012, 02:25 AM
vercammen
Continuity of fog and f/g on standard topology.
Let $f: X \rightarrow \mathbb{R}$ be continuous functions, where ($X, \tau$) is a topological space and $\mathbb{R}$ is given the standard topology.

a)Show that the function $f \cdot g : X \rightarrow \mathbb{R}$,defined by\\
$(f \cdot g)(x) = f(x)g(x)$\\
is continuous.\\

b)Let $h: X \setminus \{x \in X | g(x) = 0\}\rightarrow \mathbb{R}$ be defined by\\
$h(x) = \frac{f(x)}{g(x)}$.\\
Show that $h$ is continuous.\\

PICTURE ATTACHED!

Help!!!
• Oct 16th 2012, 11:54 PM
johnsomeone
Re: Continuity of fog and f/g on standard topology.

$\text{1) } X \rightarrow X \times X, \ x \mapsto (x,x).$

$\text{2) } X \times X \rightarrow \mathbb{R} \times \mathbb{R}, (x,y) \ \mapsto (f(x), g(y)).$

$\text{3) } \mathbb{R} \times \mathbb{R} \rightarrow \mathbb{R}, (s,t) \ \mapsto st.$
• Oct 16th 2012, 11:59 PM
vercammen
Re: Continuity of fog and f/g on standard topology.
got it. and b) is just the same only h(x) = f(x)k(x) , where k(x) = 1/g(x) , both f and k- continuous?
• Oct 17th 2012, 12:32 AM
johnsomeone
Re: Continuity of fog and f/g on standard topology.
Yes, except it's now everywhere on that subspace Y of X where g isn't 0. So everything as above, except with Y instead of X.

(Did you prove that the diagonal map X -> X x X is continuous? It's a brief but worthwhile exercise.)