Intermediate Value Theorem/Historical Note

Historical Note on Intermediate Value Theorem
This result rigorously proves the intuitive truth that:
 * if a continuous real function defined on an interval is sometimes positive and sometimes negative, then it must have the value $0$ at some point.

was the first to provide this proof in $1817$, but because of incomplete understanding of the nature of the real numbers it was not completely satisfactory.

Hence many sources refer to this as Bolzano's Theorem.

The first completely successful proof was provided by, hence its soubriquet the Weierstrass Intermediate Value Theorem.