@article{MM2_1976__18_154387,
     author = {Keijo Hild\'en},
     title = {Symmetrization of {Functions} in {Sobolev} {Spaces} and {The} {Isoperimetric} {Inequality.}},
     journal = {Manuscripta mathematica},
     pages = {215--236},
     publisher = {mathdoc},
     volume = {18},
     year = {1976},
     zbl = {0365.46031},
     url = {http://geodesic.mathdoc.fr/item/MM2_1976__18_154387/}
}