for a, c being Real
for z being Complex st a <> 0 & (4 * a) * c <= 0 & Polynom (a,0,c,0,z) = 0 & not z = (sqrt (- ((4 * a) * c))) / (2 * a) & not z = (- (sqrt (- ((4 * a) * c)))) / (2 * a) holds
z = 0