The Hadamard Parametrix on Half-Minkowski with Robin Boundary Conditions: Fundamental Solutions and Hadamard States

We address the problem of constructing fundamental solutions and Hadamard states for a Klein–Gordon field in half-Minkowski spacetime with Robin boundary conditions in d≥2 spacetime dimensions. First, using a generalization of the Robin-to-Dirichlet map exploited by Bondurant and Fulling (J Phys A: Math Theor 38:7, 2005) in dimension 2, we obtain a representation for the advanced and retarded Green operators in terms of a convolution with the kernel of the inverse Robin-to-Dirichlet map. This allows us to prove the uniqueness and support properties of the Green operators. Second, we obtain a local representation for the Hadamard parametrix that provides the correct local definition of Hadamard states in d≥2 dimensions, capturing ‘reflected’ singularities from the spacetime boundary. We show that our fundamental solutions abide by this local parametrix representation. Finally, we prove the equivalence of our local Hadamard condition and the global Hadamard condition with a wavefront set described in terms of generalized broken bi-characteristics, obtaining a Radzikowski-like theorem in half-Minkowski spacetime.