Condition for a cubic equation to have a single root If a cubic equation
$$ f(x) = ax^3+bx^2+cx+d$$
Is given, what is the condition for the equation to only have a single root (counting multiple roots as one)

Seeing that there are answers involving the derivative that miss the point:
It is about the Discriminant $ \Delta $. When $ \Delta \le 0 $ we have one real root.
See https://en.wikipedia.org/wiki/Cubic_equation#Nature_of_the_roots
