  We demonstrate in detail how the space of two-dimensional quantum field
theories can be parametrized by off-shell states of a free closed string moving
in a flat background. The dynamic equation corresponding to the condition of
conformal invariance includes an infinite number of higher order terms, and we
give an explicit procedure for their calculation. The symmetries corresponding
to equivalence relations of theories are described. In this framework we show
how to perform nonperturbative analysis in the low-energy limit and prove that
it corresponds to the Brans-Dicke theory of gravity interacting with a skew
symmetric tensor field.
