  The derivative expansion of the Wilsonian renormalization group generates
additional terms in the effective beta-functions not present in the
perturbative approach. Applied to the nonlinear sigma model, to lowest order
the vanishing of the beta-function for the tachyon field generates an equation
analogous to that found in open string field theory. Although the nonlinear
term depends on the cut-off function, this arbitrariness can be removed by a
rescaling of the tachyon field.
