@article{700224,
     title = {Structure of Green's operators and estimates for the corresponding eigenvalues},
     booktitle = {Differential Equations and Their Applications},
     series = {GDML\_Books},
     publisher = {Slovensk\'e pedagogick\'e nakladate\v lstvo},
     address = {Bratislava},
     year = {1967},
     pages = {249-272},
     zbl = {0185.42302},
     url = {http://dml.mathdoc.fr/item/700224}
}

Fichera, G. Structure of Green's operators and estimates for the corresponding eigenvalues, dans Differential Equations and Their Applications, GDML_Books,  (1967), pp. 249-272. http://gdmltest.u-ga.fr/item/700224/