## Is the $${\mathcal C}{\mathcal P}{\mathcal T}$$ norm always positive?(English)Zbl 1065.81048

### MSC:

 81Q05 Closed and approximate solutions to the Schrödinger, Dirac, Klein-Gordon and other equations of quantum mechanics 34B24 Sturm-Liouville theory 34L05 General spectral theory of ordinary differential operators 81U15 Exactly and quasi-solvable systems arising in quantum theory
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