Vacuum state of the quantum string without anomalies in any number of   dimensions
  We show that the anomalies of the Virasoro algebra are due to the asymmetric behavior of raising and lowering operators with respect to the ground state of the string. With the adoption of a symmetric vacuum we obtain a non-anomalous theory in any number of dimensions. In particular for D=4. 
C.G.Bollini and M.C.Rocca