Self-adjoint extensions by additive perturbations
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 5, Tome 2 (2003) no. 1, pp. 1-20.

Let A 𝒩 be the symmetric operator given by the restriction of A to 𝒩, where A is a self-adjoint operator on the Hilbert space and 𝒩 is a linear dense set which is closed with respect to the graph norm on D(A), the operator domain of A. We show that any self-adjoint extension A Θ of A 𝒩 such that D(A Θ )D(A)=𝒩 can be additively decomposed by the sum A Θ =A ¯+T Θ , where both the operators A ¯ and T Θ take values in the strong dual of D(A). The operator A ¯ is the closed extension of A to the whole whereas T Θ is explicitly written in terms of a (abstract) boundary condition depending on 𝒩 and on the extension parameter Θ, a self-adjoint operator on an auxiliary Hilbert space isomorphic (as a set) to the deficiency spaces of A 𝒩 . The explicit connection with both Kreĭn’s resolvent formula and von Neumann’s theory of self-adjoint extensions is given.

Classification : 47B25, 47B38, 47F05
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Posilicano, Andrea. Self-adjoint extensions by additive perturbations. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 5, Tome 2 (2003) no. 1, pp. 1-20. http://www.numdam.org/item/ASNSP_2003_5_2_1_1_0/

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