The tachyon potential in string theory

We argue that the tachyon potential in string theory is exactly given by the unstable quadratic mass term calculated perturbatively around the critical string. The argument is given in terms of the sigma model formulation. The same result follows from the exact Wilson renormalization group equations. The discrepancy with previous calculations of the tachyon potential is explained by the fact that other authors worked near the tachyon mass shell where it is impossible to distinguish a potential from derivative terms in the effective action. (orig.)