  We analyze the presumptions which lead to instabilities in theories of order
higher than second. That type of fourth order gravity which leads to an
inflationary (quasi de Sitter) period of cosmic evolution by inclusion of one
curvature squared term (i.e. the Starobinsky model) is used as an example. The
corresponding Hamiltonian formulation (which is necessary for deducing the
Wheeler de Witt equation) is found both in the Ostrogradski approach and in
another form. As an example, a closed form solution of the Wheeler de Witt
equation for a spatially flat Friedmann model and L=R\sp 2 is found. The method
proposed by Simon to bring fourth order gravity to second order can be (if
suitably generalized) applied to bring sixth order gravity to second order. In
the Erratum we show that a spatially flat Friedmann model need not be
geodesically complete even if the scale factor a(t) is positive and smooth for
all real values of the synchronized time t.
