wie kann man konkave und Konvexe Widerstände verstehen?

As already written, I’ve never heard these terms in my engineering career.
In connection with such characteristics of “resistance”, I consider to be more grossly infidel, because they actually contradict the original understanding of resistance. A diode is not a “resistance”; when current sources are added, it will be abstructed.
Such designations serve at best as a model for certain, not proportional or even nonlinear, and not positive Conditions of U and I. They must then always be described by the characteristic, the expression alone says nothing.
As I said, I don’t see any sense in it. They are simply U-I characteristics, but no resistances. In what course do you get such things?

Here the pictures are still rotated: