# Counter Example for Theorem about Monotonic Functions

• Apr 19th 2011, 06:45 PM
jameselmore91
Counter Example for Theorem about Monotonic Functions
I'm trying (and failing) to find a counterexample to the following false theorem:

For all functions \$\displaystyle f,g\$ from Reals to Reals, if \$\displaystyle g \circ f\$ is monotonic, then at least one of \$\displaystyle f\$ or \$\displaystyle g\$ is a monotonic function.
• Apr 19th 2011, 08:38 PM
TKHunny
How about f(x) = asin(1/x) and g(x) = 1/x? Or are this f and g monotonic?
• Apr 20th 2011, 08:17 AM
jameselmore91
I guess this will work, but since asin(1/x) is undefined from x = (-1, 1) it technically isn't a function on the reals. It's only a function from R - (-1, 1) to R.

If I come up with no other examples this will do, but I feel there should be an example with a true function from R to R
• Apr 20th 2011, 01:39 PM
awkward
How about f(x) = 1 if x is rational, -1 if x is irrational, and g(x) = x^2?
• Apr 20th 2011, 08:02 PM
TKHunny
I was tryng to come up with something at least mostly continuous.