see also: Elf[?] (1971-1976), Rainbow (1974-1979), Black Sabbath (1979-1983; returns 1991) and Dio (1982-present)
Discography:
With Rainbow
With Black Sabbath
With Dio
... are real. (Geometrically, this means that the parabola intersects the x-axis in two points.) If the discriminant is negative, then there are two different solutions x, ...