  In this paper, we give a sufficient and necessary condition (witness) for the
sudden change of geometric quantum discord by considering mathematical
definition of the discontinuity of a function. Based on the witness, we can
find out various sudden changes of quan- tum correlation by considering both
the Markovian and the non-Markovian cases. In particular, we can accurately
find out critical points of the sudden changes even though they are not quite
obvious in the graphical representation. In addition, one can also find that
sudden change of quantum correlation, like the frozen quantum correlation,
strongly depends on the choice of the quantum correlation measure.
