# question on a proof

• Oct 31st 2009, 08:16 PM
dannyboycurtis
question on a proof
I need a little confirmation on the following proof:
The problem is this:
For an interval I, with f:I -> R where f is strictly increasing with f(I) an interval, prove that f^(-1):R -> I is strictly increasing.

My proof is this:
Because f is strictly increasing, for y,z exists in I such that y<z,
f(y)<f(z).
Let y,z exist in I. Then f(y),f(z) exist in R. and f(y) < f(z).
but f^(-1)[f(y)]<f^(-1)[f(z)] which implies y<z and thus f^(-1) is strictly increasing.
This proof seems trivial to me, which makes me think I did something wrong. I possibly assumed too much in my proof, not sure. Any ideas?
• Oct 31st 2009, 09:55 PM
aukie
Hello

I suffer from similar habbits. Always assuming things are complicated. Your proof is correct in its simplicity. You are absolutely right to assume the existence of \$\displaystyle f^{-1}\$, its given in the problem statement. Hence you may use the identity function \$\displaystyle f^{-1}(f(x)) = x\$, by the definition of the inverse.

In fact just to reassure you that your proof makes sense, remember that f is the inverse of the function \$\displaystyle f^{-1}\$. And a function only has an inverse if it is strictly increasing or decreasing (a standard theorem from set theory). You have of course proved \$\displaystyle f^{-1} \$ is strictly increasing. And thus the inverse of \$\displaystyle f^{-1}\$ exists. namely, f.