1. Aigner M. Combinatorial Theory., Springer, 1979.
2. Bernstein I., Gelfand I., Ponomarev V. Uspechi Mat.Nauk, 1973, 28, N 2.
3. Birkhoff G. Lattice Theory, Providence, 1967.
4. Gabriel P., Manuscr. Math. 1972, N 6, 71–103.
5. Gelfand I.M., Dokl.Akad.Nauk SSSR, 1986, 288, N 2.