Gauge Invariant Exact Renormalization Group and Perfect Actions in the Open Bosonic String Theory

The exact renormalization group is applied to the world sheet theory describing bosonic open string backgrounds to obtain the equations of motion for the fields of the open string. Using loop variable techniques the equations can be constructed to be gauge invariant. Furthermore they are valid off the (free) mass shell. This requires keeping a finite cutoff. Thus we have the interesting situation of a scale invariant world sheet theory with a finite world sheet cutoff. This is possible because there are an infinite number of operators whose coefficients can be tuned. This is in the same sense that “perfect actions” or “improved actions” have been proposed in lattice gauge theory to reproduce the continuum results even while keeping a finite lattice spacing.