@article{MZ_1987__194_173821,
     author = {Peter Hess},
     title = {An {Isoperimetric} {Inequality} for the {Principal} {Eigenvalue} of a {Periodic-Parabolic} {Problem.}},
     journal = {Mathematische Zeitschrift},
     pages = {121--126},
     publisher = {mathdoc},
     volume = {194},
     year = {1987},
     zbl = {0629.35050},
     url = {http://geodesic.mathdoc.fr/item/MZ_1987__194_173821/}
}