We prove Gaussian upper bounds for kernels associated with non-symmetric, non-autonomous second order parabolic operators of divergence form subject to various boundary conditions. The growth of the kernel in time is determined by the boundary conditions and geometric properties of the domain. The theory gives a unified treatment for Dirichlet, Neumann and Robin boundary conditions, and the existence of a Gaussian type bound is essentially reduced to verifying some properties of the Hilbert space in the weak formulation of the problem.
AMS Subject Classification (1991): Primary: 35K20
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