A symmetry for vanishing cosmological constant in an extra dimensional   toy model
  We introduce a symmetry principle that forbids a bulk cosmological constant in six and ten dimensions. Then the symmetry is extended so that it insures absence of 4-dimensional cosmological constant induced by the six dimensional curvature scalar, at least, for a class of metrics. A small cosmological constant may be induced by breaking of the symmetry by a small amount. 
Recai Erdem