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Sylow theorems
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=== Statement === Collections of subgroups that are each maximal in one sense or another are common in group theory. The surprising result here is that in the case of <math>\operatorname{Syl}_p(G)</math>, all members are actually [[group isomorphism|isomorphic]] to each other and have the largest possible order: if <math>|G|=p^nm</math> with <math>n > 0</math> where {{mvar|p}} does not divide {{mvar|m}}, then every Sylow {{mvar|p}}-subgroup {{mvar|P}} has order <math>|P| = p^n</math>. That is, {{mvar|P}} is a {{mvar|p}}-group and <math>\text{gcd}(|G:P|, p) = 1</math>. These properties can be exploited to further analyze the structure of {{mvar|G}}. The following theorems were first proposed and proven by Ludwig Sylow in 1872, and published in ''[[Mathematische Annalen]]''. {{math theorem|note=1|For every [[prime factor]] {{mvar|p}} with [[multiplicity of a prime factor|multiplicity]] {{mvar|n}} of the order of a finite group {{mvar|G}}, there exists a Sylow [[p-group|{{mvar|p}}-subgroup]] of {{mvar|G}}, of order ''<math>p^n</math>''.}} The following weaker version of theorem 1 was first proved by [[Augustin-Louis Cauchy]], and is known as [[Cauchy's theorem (group theory)|Cauchy's theorem]]. {{math theorem|name=Corollary|Given a finite group {{mvar|G}} and a prime number {{mvar|p}} dividing the order of {{mvar|G}}, then there exists an element (and thus a cyclic subgroup generated by this element) of order {{mvar|p}} in ''{{mvar|G}}''.{{r|Fraleigh_2004_322}}}} {{math theorem|note=2|Given a finite group {{mvar|G}} and a prime number {{mvar|p}}, all Sylow {{mvar|p}}-subgroups of {{mvar|G}} are [[conjugacy class|conjugate]] to each other. That is, if {{mvar|H}} and {{mvar|K}} are Sylow {{mvar|p}}-subgroups of {{mvar|G}}, then there exists an element <math>g \in G</math> with <math>g^{-1}Hg = K</math>.}} {{math theorem|note=3|Let {{mvar|p}} be a prime factor with multiplicity {{mvar|n}} of the order of a finite group {{mvar|G}}, so that the order of {{mvar|G}} can be written as <math>p^nm</math>, where <math>n > 0</math> and {{mvar|p}} does not divide {{mvar|m}}. Let <math>n_p</math> be the number of Sylow {{mvar|p}}-subgroups of {{mvar|G}}. Then the following hold: * <math>n_p</math> divides {{mvar|m}}, which is the [[index of a subgroup|index]] of the Sylow {{mvar|p}}-subgroup in {{mvar|G}}. * <math>n_p \equiv 1 \pmod{p}</math> * <math>n_p = |G:N_G(P)|</math>, where {{mvar|P}} is any Sylow {{mvar|p}}-subgroup of {{mvar|G}} and <math>N_G</math> denotes the [[normalizer]]. }}
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