  We show that a simple and intuitive three-parameter equation fits remarkably
well the evolution of the gross domestic product (GDP) in current and constant
dollars of many countries during times of recession and recovery. We then argue
that this equation is the response function of the economy to isolated shocks,
hence that it can be used to detect large and small shocks, including those
which do not lead to a recession; we also discuss its predictive power.
Finally, a two-sector toy model of recession and recovery illustrates how the
severity and length of recession depends on the dynamics of transfer rate
between the growing and failing parts of the economy.
