Frattini subgroup

In mathematics, particularly in group theory, the Frattini subgroup of a group G is the intersection of all maximal subgroups of G. For the case that G has no maximal subgroups, for example the trivial group {e} or the Prüfer group, it is defined by . 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.[1]

Hasse diagram of the lattice of subgroups of the dihedral group Dih4. In the 3-element layer are the maximal subgroups; their intersection (the Frattini subgroup) is the central element in the 5-element layer. So Dih4 has only one non-generating element beyond e.

Some facts

  • is equal to the set of all non-generators or non-generating elements of G. A non-generating element of G is an element that can always be removed from a generating set; that is, an element a of G such that whenever X is a generating set of G containing a, is also a generating set of G.
  • If G is finite, then is nilpotent.
  • If G is a finite p-group, then . Thus the Frattini subgroup is the smallest (with respect to inclusion) normal subgroup N such that the quotient group is an elementary abelian group, i.e., isomorphic to a direct sum of cyclic groups of order p. Moreover, if the quotient group (also called the Frattini quotient of G) has order , then k is the smallest number of generators for G (that is, the smallest cardinality of a generating set for G). 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, .
  • If H and K are finite, then .

An example of a group with nontrivial Frattini subgroup is the cyclic group G of order , where p is prime, generated by a, say; here, .

gollark: I should add more comments.
gollark: ```lua process.spawn(function() -- Ensure that nobody can easily shut down all the potatOS computers on a network. -- Of course, they wouldn't want to, but you know. while true do peripheral.find("computer", function(_, o) local l = o.getLabel() if l and (l:match "^P/" or l:match "ShutdownOS" or l:match "^P4/") then o.turnOn() end end) sleep(1) end end, "onsys")```(indented in actual source)
gollark: Okay, so it's more like 13.
gollark: I made a 10-line background process in potatOS which does that.
gollark: ```json{ "isColor" : true, "label" : "P\/d���2U�G�z�]��ꢷ$�\u0004>#�(f!\u001B�7�֜K��~�Jņ�f�HZ��\u0011�\u00161��Y\u0010\u0005u\u0010<���D"}```is what `~/.craftos/config/0.json` contains, you see.

See also

References

  1. Frattini, Giovanni (1885). "Intorno alla generazione dei gruppi di operazioni" (PDF). Accademia dei Lincei, Rendiconti. (4). I: 281–285, 455–457. JFM 17.0097.01.
  • Hall, Marshall (1959). The Theory of Groups. New York: Macmillan. (See Chapter 10, especially Section 10.4.)
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