The blue squares are inflection points. In mathematics, particularly in calculus, a stationary point or critical point of a differentiable function of one variable is a point of the domain of the function where the derivative is zero (equivalently, the slope of the graph at that point is zero).f the function is twice differentiable, the stationary points that are not turning points are horizontal inflection points. For example the function has a stationary point at x=0, which is also an inflection point, but is not a turning point.