Krein space operator-theoretic methods are used to prove that the operator (sgn x) d²/dx² is similar to a selfadjoint operator in the Hilbert space L²(R).
Proceedings of the American Mathematical Society
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First published in "Proceedings of the American Mathematical Society" in 1995, published by the American Mathematical Society.
Ćurgus, Branko and Najman, Branko, "The Operator (sgn x) d²/dx² is Similar to a Selfadjoint Operator in L² (R)" (1995). Mathematics. Paper 10.