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Sylow intersections and Frobenius ratios

  • Wolfgang Knapp,
  • Peter Schmid

摘要

Let G be a finite group and p a prime dividing its order |G|, with p-part \(|G|_p\) | G | p , and let \(G_p\) G p denote the set of all p-elements in G. A well known theorem of Frobenius tells us that \(f_p(G)=|G_p|/|G|_p\) f p ( G ) = | G p | / | G | p is an integer. As \(G_p\) G p is the union of the Sylow p-subgroups of G, this Frobenius ratio \(f_p(G)\) f p ( G ) evidently depends on the number \(s_p(G)=|\textrm{Syl}_p(G)|\) s p ( G ) = | Syl p ( G ) | of Sylow p-subgroups of G and on Sylow intersections. One knows that \(s_p(G)=1+kp\) s p ( G ) = 1 + k p and \(f_p(G)=1+\ell (p-1)\) f p ( G ) = 1 + ( p - 1 ) for nonnegative integers \(k, \ell \) k , , and that \(f_p(G)<s_p(G)\) f p ( G ) < s p ( G ) unless G has a normal Sylow p-subgroup. In order to get lower bounds for \(f_p(G)\) f p ( G ) we, study the permutation character \({\pi }={\pi }_p(G)\) π = π p ( G ) of G in its transitive action on \(\textrm{Syl}_p(G)\) Syl p ( G ) via conjugation (Sylow character). We will get, in particular, that \(f_p(G)\ge s_p(G)/r_p(G)\) f p ( G ) s p ( G ) / r p ( G ) where \(r_p(G)\) r p ( G ) denotes the number of P-orbits on \(\textrm{Syl}_p(G)\) Syl p ( G ) for any fixed \(P\in \textrm{Syl}_p(G)\) P Syl p ( G ) . One can have \(\ell \ge k\ge 1\) k 1 only when P is irredundant for \(G_p\) G p , that is, when P is not contained in the union of the \(Q\ne P\) Q P in \(\textrm{Syl}_p(G)\) Syl p ( G ) and so \(\widehat{P}=\bigcup _{Q\ne P}(P\cap Q)\) P ^ = Q P ( P Q ) a proper subset of P. We prove that \(\ell \ge k\) k when \(|\widehat{P}|\le |P|/p\) | P ^ | | P | / p .