The classes of partially ordered groups
Compositio Mathematica, Tome 9 (1951) , pp. 130-140.
@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},
     zbl = {0043.25602},
     mrnumber = {45740},
     language = {en},
     url = {www.numdam.org/item/CM_1951__9__130_0/}
}
Loonstra, F. The classes of partially ordered groups. Compositio Mathematica, Tome 9 (1951) , pp. 130-140. http://www.numdam.org/item/CM_1951__9__130_0/