FLOWS AND INVARIANCE FOR DEGENERATE ELLIPTIC OPERATORS

Abstract

AbstractLetSbe a sub-Markovian semigroup onL2(ℝd) generated by a self-adjoint, second-order, divergence-form, elliptic operatorHwithW1,∞(ℝd) coefficientsckl, and let Ω be an open subset of ℝd. We prove that ifeither C∞c(ℝd) is a core of the semigroup generator of the consistent semigroup onLp(ℝd) for somep∈[1,∞]  or Ω has a locally Lipschitz boundary, thenSleavesL2(Ω) invariant if and only if it is invariant under the flows generated by the vector fields ∑dl=1ckl∂lfor allk. Further, for allp∈[1,2] we derive sufficient conditions on the coefficients for the core property to be satisfied. Then by combination of these results we obtain various examples of invariance in terms of boundary degeneracy both for Lipschitz domains and domains with fractal boundaries.