@article{ARM_2005_41_2_a1,
     author = {Mikulski, W. M.},
     title = {On naturality of the {Helmholtz} operator},
     journal = {Archivum mathematicum},
     pages = {145--149},
     year = {2005},
     volume = {41},
     number = {2},
     mrnumber = {2164663},
     zbl = {1112.58004},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_2005_41_2_a1/}
}