Editing Kummer theory (section)

{{Short description|Theory in abstract algebra}}
In [[abstract algebra]] and [[number theory]], '''Kummer theory''' provides a description of certain types of [[field extension]]s involving the [[adjunction (field theory)|adjunction]] of ''n''th roots of elements of the base [[field (mathematics)|field]]. The theory was originally developed by [[Ernst Kummer|Ernst Eduard Kummer]] around the 1840s in his pioneering work on [[Fermat's Last Theorem]]. The main statements do not depend on the nature of the field – apart from its [[characteristic of a field|characteristic]], which should not divide the integer ''n'' – and therefore belong to abstract algebra. The theory of cyclic extensions of the field ''K'' when the characteristic of ''K'' does divide ''n'' is called [[Artin–Schreier theory]].

Kummer theory is basic, for example, in [[class field theory]] and in general in understanding [[abelian extension]]s; it says that in the presence of enough roots of unity, cyclic extensions can be understood in terms of extracting roots. The main burden in class field theory is to dispense with extra roots of unity ('descending' back to smaller fields); which is something much more serious.