2 citations to https://www.mathnet.ru/eng/dan1193
  1. Proc. Steklov Inst. Math., 260 (2008), 68–89  mathnet  crossref  mathscinet  zmath  isi  elib
  2. V. I. Burenkov, P. D. Lamberti, M. Lanza de Cristoforis, “Spectral Stability of Nonnegative Self-Adjoint Operators”, Journal of Mathematical Sciences, 149:4 (2008), 1417–1452  mathnet  crossref  mathscinet