I want to bring up my issue one more time to finally solve it! I want to understand how it works.
Here is what I have:
I want to calculate the left derivate.
I got from the integral which is (1)
The second inequality I get is from the advice from girdav which was to consider
This inequality can be put to and it leads to (2)
I got (1) and (2). How can I use these two inequalities for the left derivate? If I can replace the in (1) by something else which is bigger than k I would get the same derivate.
My conclusions should be correct until here but I donīt see how I can proceed from there.
I need a hint and I would be very thankful for that little hint to finally finish the work.