Extremum Command
From GeoGebra Manual
This command differs among variants of English:

 Extremum( <Polynomial> )
 Yields all local extrema of the polynomial function as points on the function graph.
 Example:
Extremum(x³ + 3x²  2x + 1)
creates local extrema (0.29, 0.70) and (2.29, 9.30) and shows them in the Graphics View.
 Extremum( <Function>, <Start xValue>, <End xValue> )
 Calculates (numerically) the extremum of the function in the open interval ( <Start xValue>, <End xValue> ).
 Example:
Extremum((x⁴  3x³  4x² + 4) / 2, 0, 5)
creates local extremum (2.93, 16.05) in the given interval and shows it in the Graphics View.
 Note: The function should be continuous in [ <Start xValue>, <End xValue> ], otherwise false extrema near discontinuity might be calculated.