MHB Prove Monotony of Function: $f$ Strictly Decreasing

  • Thread starter Thread starter awsomeman
  • Start date Start date
  • Tags Tags
    Function
awsomeman
Messages
1
Reaction score
0
Let $f$ be differentiable from $(-\inf,0)$ to $(0,\inf)$ and let $f'(x)<0$ for all real numbers except 0 and $f'(0)=0$. Prove that f is strictly decreasing.
 
Physics news on Phys.org
You might want to begin by stating the definition of a decreasing function. Then consider some examples.
 
I would use "proof by contradiction". Suppose f is NOT strictly decreasing. Then there exist a, b, b> a, such that f(b)\ge f(a). So $f(b)- f(a)\ge 0$. Since b> a, b- a> 0 so $\frac{f(b)- f(a)}{b- a}\ge 0$. Now use the "mean value" property.
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.
Back
Top