Frattini subgroup

File:Dih4 subgroups (cycle graphs).svg

Hasse diagram of the lattice of subgroups of the dihedral group Dih₄. In the second row are the maximal subgroups; their intersection (the Frattini subgroup) is the central element in the third row. So Dih₄ has only one non-generating element beyond e.

In mathematics, particularly in group theory, the Frattini subgroup <math>\Phi(G)</math> of a group Template:Mvar is the intersection of all maximal subgroups of Template:Mvar. For the case that Template:Mvar has no maximal subgroups, for example the trivial group {e} or a Prüfer group, it is defined by <math>\Phi(G)=G</math>. It is analogous to the Jacobson radical in the theory of rings, and intuitively can be thought of as the subgroup of "small elements" (see the "non-generator" characterization below). It is named after Giovanni Frattini, who defined the concept in a paper published in 1885.<ref>Template:Cite journal</ref>

Some facts

• <math>\Phi(G)</math> is equal to the set of all non-generators or non-generating elements of Template:Mvar. A non-generating element of Template:Mvar is an element that can always be removed from a generating set; that is, an element a of Template:Mvar such that whenever Template:Mvar is a generating set of Template:Mvar containing a, <math>X \setminus \{a\}</math> is also a generating set of Template:Mvar.
• <math>\Phi(G)</math> is always a characteristic subgroup of Template:Mvar; in particular, it is always a normal subgroup of Template:Mvar.
• If Template:Mvar is finite, then <math>\Phi(G)</math> is nilpotent.
• If Template:Mvar is a finite p-group, then <math>\Phi(G)=G^p [G,G]</math>. Thus the Frattini subgroup is the smallest (with respect to inclusion) normal subgroup N such that the quotient group <math>G/N</math> is an elementary abelian group, i.e., isomorphic to a direct sum of cyclic groups of order p. Moreover, if the quotient group <math>G/\Phi(G)</math> (also called the Frattini quotient of Template:Mvar) has order <math>p^k</math>, then k is the smallest number of generators for Template:Mvar (that is, the smallest cardinality of a generating set for Template:Mvar). In particular a finite p-group is cyclic if and only if its Frattini quotient is cyclic (of order p). A finite p-group is elementary abelian if and only if its Frattini subgroup is the trivial group, <math>\Phi(G)=\{e\}</math>.
• If Template:Mvar and Template:Mvar are finite, then <math>\Phi(H\times K)=\Phi(H) \times \Phi(K)</math>.

An example of a group with nontrivial Frattini subgroup is the cyclic group Template:Mvar of order <math>p^2</math>, where p is prime, generated by a, say; here, <math>\Phi(G)=\left\langle a^p\right\rangle</math>.

See also

• Fitting subgroup
• Frattini's argument
• Socle

References

• Template:Cite book (See Chapter 10, especially Section 10.4.)