[4] J. Rachůnek:
Prime subgroups of ordered groups. Czech. Math. J. 24 (99), 1974, 541-551.
MR 0357274
[5] F. Šik: On the set of lattice ordered groups. Czech. Math. J. 6 (1956), 1-25.
[6] F. Šik: A characterization of polarities the lattice of polars of which is Boolean. 1974 (un- published).
[7] B. Šmarda:
Polars and x-ideals in semigroups. Mat. čas. 1 (26), 1976, 31-37.
MR 0427506
[8] В. Šmarda:
Polars on closure spaces. Arch. Math. 2, ХIII (1977), 117-124.
MR 0460192