Bernard Bolzano (1781-1848) presents Bolzano’s theorem: a continuous function which is negative at one point and positive at another point must be zero for at least one point in between.
Editors. "Bolzanos theorem". The Literary Encyclopedia. First published 01 May 2009; last revised 30 November -1.
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6245, accessed 26 April 2015.]