@article{ Solr-acta32903, title = {Essential spectrum of systems of singular differential equations}, series = {Acta scientiarum mathematicarum}, author = {Ibrogimov Orif O.}, year = {2013}, url = {http://acta.bibl.u-szeged.hu/32903/1/math_079_numb_003_004_423-465.pdf} }