# Contraction Mappings without a fixed point

• Sep 14th 2007, 09:48 AM
Contraction Mappings without a fixed point
Show that the following mapping $f:X \rightarrow X$ do not have a fixed point and explain why the Contraction Mapping Principle is not contradicted.
Problem: Let X=R and f(x) = x+1 for all x in X.

My Solution:
f do not have a fixed point because x does not equal to x+1 for any real number x.

d(f(x),f(y)) = |(x+1) - (y+1)| = |x-y|,
and d(x,y) = |x-y|.

So d(f(x),f(y)) = d(x,y), now I know I have to find a constant c with 0 <= C < 1 for the Contraction Mapping Principle to hold. But I can't find that c.

• Sep 14th 2007, 12:57 PM
CaptainBlack
Quote:

Problem: Let X=R and f(x) = x+1 for all x in X.

Show that the mapping doesn't have a fixed point, and the Contraction Mapping Theorem is not contradicted.

Solution: d(f(x),f(y)) = |(x+1) - (y+1)| = |x-y|,
and d(x,y) = |x-y|.

So d(f(x),f(y)) = d(x,y), now I know I have to find a constant c with 0 <= C < 1 for the Contraction Mapping Principle to hold. But I can't find that c.

2. If there was no fixed point show that at least one of the conditions
of the the contraction mapping theorem is violated.

RonL
• Sep 14th 2007, 01:34 PM
I think I didn't word the question correctly, let me edit.

"Show that the following mapping $f:X \rightarrow X$ do not have a fixed point and explain why the Contraction Mapping Principle is not contradicted."

So if I understand this question correctly, I have to show the mapping do not have a fixed point, but yet it is still a contraction. (I guess because it is not in a complete space?)

And yeah, I forgot to put the first part of my answer on there, it is added now.
• Sep 14th 2007, 01:51 PM
topsquark
Quote:

I think I didn't word the question correctly, let me edit.

"Show that the following mapping $f:X \rightarrow X$ do not have a fixed point and explain why the Contraction Mapping Principle is not contradicted."

So if I understand this question correctly, I have to show the mapping do not have a fixed point, but yet it is still a contraction. (I guess because it is not in a complete space?)

And yeah, I forgot to put the first part of my answer on there, it is added now.

No, "the Contraction Mapping Principle is not contradicted." You need to show why the mapping does not have a fixed point and why there is no contradiction.

-Dan
• Sep 14th 2007, 06:12 PM