- #1
cap.r
- 67
- 0
Homework Statement
Suppose that X is a metric Space and supposed that the mapping f:R-->X is continuous. Let C be a closed subset of X and let f(x) belong to C if x is rational. Prove that f(R) is in C.
Homework Equations
Def. a set X is closed if every convergent sequence in X converges within X.
Def. a mapping f:X-->Y is continuous at a point p in X provided that whenever a sequence {pk} in X converges to p, the image sequence {f(pk)} converges to f(p)
The Attempt at a Solution
let {uk} be in Q. and uk-->u in Q. then the continuity of f implies f(uk)-->f(u), f(uk),f(u) are in C since uk,u are in Q.
Now since f is a cont mapping and f(uk) must converge to f(u) continuously, f can't just jump over all the irrationals between f(uk) and f(u). so the denseness of irrationals forces f(R) to be in C.
this last part is in no way formal but makes sense to me... can someone please help me formalize this proof. and fix my logic if i am going wrong anywhere.