Fortifications are militaryconstructions designed for defensivewarfare. They have been used for many thousands of years, in a variety of increasingly complex designs. The practice of improving an area's defense is also known as "fortification". Many military installations are known as forts, although they are not always "fortified".
... because it discriminates between three qualitatively different cases:
If the discriminant is zero, then there is a repeated solution x, and this solution is real. ...