String field equations from the generalized sigma model

We propose a new approach for deriving the string field equations from a general sigma model on the world-sheet, This approach leads to an equation which combines some of the attractive features of both the renormalization group method and the covariant beta function treatment of the massless excitations. It has the advantage of being covariant under a very general set of both local and non-local transformations in the field space. We apply it to the tachyon, massless and first massive level, and show that the resulting field equations reproduce the correct spectrum of a left-right symmetric closed bosonic string. (C) 1997 Elsevier Science B.V.