Cojuhari, P.Gheondea, A.2016-02-082016-02-0820050022-247Xhttp://hdl.handle.net/11693/24077We introduce the notion of induced Hilbert spaces for positive unbounded operators and show that the energy spaces associated to several classical boundary value problems for partial differential operators are relevant examples of this type. The main result is a generalization of the Krein-Reid lifting theorem to this unbounded case and we indicate how it provides estimates of the spectra of operators with respect to energy spaces. © 2004 Elsevier Inc. All rights reserved.EnglishBoundary value problemsEnergy spaceInduced Hilbert spaceLifting of operatorsSpectrumArticle10.1016/j.jmaa.2004.09.0481096-0813