@article{700159,
     title = {A posteriori estimations of approximate solutions for some types of boundary value problems},
     booktitle = {Equadiff 6},
     series = {GDML\_Books},
     publisher = {J. E. Purkyn\v e University, Department of Mathematics},
     address = {Brno},
     year = {1986},
     pages = {[373]-378},
     mrnumber = {MR0877151},
     zbl = {0609.35001},
     url = {http://dml.mathdoc.fr/item/700159}
}

Kodnár, Rudolf. A posteriori estimations of approximate solutions for some types of boundary value problems, dans Equadiff 6, GDML_Books,  (1986), pp. [373]-378. http://gdmltest.u-ga.fr/item/700159/