Y by
Suppose a function
satisfies
for all real
. Then, we have
We also have
Adding the first two inequalities and using the third, we have
which implies
. But this means
is bounded above which is absurd -- take the identity function. I feel I am making some blithely stupid mistake, but I can't see what. Any help is appreciated 




![\[f(\tfrac{x+y}{2}) + f(\tfrac{x-y}{2}) \ge 2f(x).\]](http://latex.artofproblemsolving.com/5/3/d/53d5e6a96b03a5c4a05a9da1945cbaa81829271e.png)
![\[2f(x) + f(y)+f(-y) \ge 2(f(\tfrac{x+y}{2}) + f(\tfrac{x-y}{2})) \ge 4f(x),\]](http://latex.artofproblemsolving.com/a/9/c/a9c9255deeb88e31603a55d6cf32fbf864ca0758.png)



This post has been edited 3 times. Last edited by Sedro, Jun 17, 2024, 3:57 PM