@article{MM2_1977__20_154434,
     author = {Ralf Kern},
     title = {Mulitply {Periodic} {Real} {Functions} and {Their} {Period} {Sets.}},
     journal = {Manuscripta mathematica},
     pages = {153--176},
     publisher = {mathdoc},
     volume = {20},
     year = {1977},
     zbl = {0369.26005},
     url = {http://geodesic.mathdoc.fr/item/MM2_1977__20_154434/}
}