@article{CM_1951__9__130_0,
     author = {Loonstra, F.},
     title = {The classes of partially ordered groups},
     journal = {Compositio Mathematica},
     pages = {130--140},
     publisher = {Kraus Reprint},
     volume = {9},
     year = {1951},
     mrnumber = {45740},
     zbl = {0043.25602},
     language = {en},
     url = {http://www.numdam.org/item/CM_1951__9__130_0/}
}