  The representation theory of the Virasoro algebra in the case of a
logarithmic conformal field theory is considered. Here, indecomposable
representations have to be taken into account, which has many interesting
consequences. We study the generalization of null vectors towards the case of
indecomposable representation modules and, in particular, how such logarithmic
null vectors can be used to derive differential equations for correlation
functions. We show that differential equations for correlation functions with
logarithmic fields become inhomogeneous.
