Considering the figure below, I wonder what the red circle is called in math/geometry. It is neither min/max nor inflection point (the whole math I know). I did a search an found nothing as I do not know the right vocabulary. Actually, the blue curve shows the possible rate of two parameters (e.g., a/b). At the red circle, my function is reached to its optimal value. I wonder how can I find the red circle?
Best,
