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.
Citation:
Editors. "Bolzanos theorem".
The Literary Encyclopedia. First published 01 May 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6245, accessed 19 June 2013.]