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The exponent of a group , denoted , is equivalently
the least natural number such that , the identity element, for all ,
hence the least common multiple of the orders of the group elements.
Here “least” should, by all rights, refer to the divisibility order on , where means divides . Notice that with this poset structure, is a complete lattice with bottom element and top element ; under this convention, the exponent always exists as the least common multiple of the orders of elements, which is their join (supremum) in the lattice, and is if (for example) is a torsion-free group.
If is a finite group of order , then divides since for all .
If is a finite abelian group and , then is cyclic.
Of course can’t be , so here has a prime factorization . Since is the least common multiple of the orders of elements, the exponent is the maximum multiplicity of occurring in orders of elements; any element realizing this maximum will have order divisible by , and some power of that element will have order exactly . Then will have order by the following lemma and induction, so that powers of exhaust all elements of , i.e., generates as desired.
If are relatively prime and has order and has order in an abelian group, then has order .
Suppose . For some we have , and so . It follows that divides . Similarly divides , so divides , as desired.
If is a finite group, then the prime factors of coincide with the prime factors of .
Since divides , every prime factor of is a factor of . If is a prime factor of , then by the Cauchy group theorem, has an element of order , and thus divides .
If , then is abelian and is a vector space over the field .
Similarly, if is abelian and its exponent is a prime , then is a vector space over .
A finitely generated group of exponent , , or must be finite. On the other hand, it is not known whether a group of exponent generated by elements must be finite.
These last facts are part of the lore of the celebrated Burnside problem. The free Burnside group of exponent with generators is presented by , and one formulation of the original Burnside problem was whether is finite. It is now known that the answer is negative for all and odd , and there is a similar result for certain even as well.
For a fixed prime , a Tarski monster for is a finitely generated infinite group where every proper nontrivial subgroup is cyclic of order . Such monsters exist for sufficiently large , forming a class of dramatic counterexamples to the Burnside conjecture; they are of course of exponent .
It was shown by Efim Zelmanov that there are only finitely many finite groups with generators and exponent (restricted Burnside problem). It was largely on the strength of this work that he was awarded the Fields Medal (1994).
See also
Wikipedia, Burnside problem, web
Last revised on September 2, 2021 at 08:38:07. See the history of this page for a list of all contributions to it.