is: a test to determine the location and nature of local extrema of a given function
involves: (a) finding the points at which 
(b) investigating the behaviour of the sign of 
in the neighbourhood of these points. If 
and changes from positive to negative at 
then there is a local maximum at 
If and changes from negative to positive at 
then there is a local minimum at If but does not change sign at then further investigation is required. [M4.4]
See stationary points and graph sketching in the Maths handbook.
Copyright 1997, The Open University