J. Behrndt and M. Langer,
Elliptic operators, Dirichlet-to-Neumann maps and quasi boundary triples,
in "Operator Methods for Boundary Value Problems",
London Math. Soc. Lecture Note Series, vol. 404,
Cambridge University Press, Cambridge, 2012, pp. 121–160

Abstract:
The notion of quasi boundary triples and their Weyl functions is reviewed and applied to self-adjointness and spectral problems for a class of elliptic, formally symmetric, second order partial differential expressions with variable coefficients on bounded domains.

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