@article{700056,
     title = {Interior error estimates of projection methods},
     booktitle = {Proceedings of Equadiff III},
     series = {GDML\_Books},
     publisher = {Univ. J. E. Purkyn\v e - P\v r\'\i rodov\v edeck\'a fakulta},
     address = {Brno},
     year = {1973},
     pages = {235-239},
     mrnumber = {MR0359361},
     zbl = {0357.65090},
     url = {http://dml.mathdoc.fr/item/700056}
}

Nitsche, Joachim A. Interior error estimates of projection methods, dans Proceedings of Equadiff III, GDML_Books,  (1973), pp. 235-239. http://gdmltest.u-ga.fr/item/700056/