They are certainly "low" points. In spirit, they are relatively extreme, but they don't fit the definition of relative extremum (no derivative, ... ... <看更多>

The relative extrema of a function appear where f ′ ( x ) changes from positive to negative or from negative to positive. Since ... ... <看更多>

https://goo.gl/JQ8NysA video which shows how to find the relative extrema of a function. In this video the second derivat... ... <看更多>