@article{702188,
     title = {On a certain extension of the maximum principle},
     booktitle = {Differential Equations and Their Applications},
     series = {GDML\_Books},
     publisher = {Publishing House of the Czechoslovak Academy of Sciences},
     address = {Prague},
     year = {1963},
     pages = {223-228},
     mrnumber = {MR0173860},
     zbl = {0145.36703},
     url = {http://dml.mathdoc.fr/item/702188}
}

Výborný, R. On a certain extension of the maximum principle, dans Differential Equations and Their Applications, GDML_Books,  (1963), pp. 223-228. http://gdmltest.u-ga.fr/item/702188/