 introduced by Keigher. We focus on the PF and PP-properties in
introduced by Keigher. We focus on the PF and PP-properties in  and
its sub-ring
 and
its sub-ring  of Hurwitz polynomials.
 of Hurwitz polynomials.
Key Words: Hurwitz series and polynomials; PP-ring; PF-ring.
2000 Mathematics Subject Classification: Primary: 13F25;
Secondary: 13B25; 13C12.
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