Instanton solutions in gravitational theories with nonlinear lagrangian

We obtain gravitational instanton solutions in the Einstein theory coupled with a scalar matter field that is conformally equivalent to gravitational theories with nonlinear lagrangian which are polynomial in the scalar curvature and contain a cosmological constant both for the quadratic and the large curvature limit cases. We also argue that the necessary condition for the existence of these solutions may be satisfied for nonlinear theories whose lagrangian depends on the full Ricci tensor. The implications of these solutions to the mechanism for vanishing of the low-energy cosmological constant are considered. (orig.)