  This paper has been withdrawn due to errors found by authors. A correct
version of this paper is published in:
  Tomasz M. Rusin, Wlodek Zawadzki, "Quantum theory of symmetric screening in
the Hartree approximation", Phys. stat. sol. (b) 243, 1286-1295 (2006), DOI:
10.1002/pssb.200541204.
