Cyclotomic field

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In number theory, a cyclotomic field is a number field obtained by adjoining a complex root of unity to ${\displaystyle \mathbb {Q} }$, the field of rational numbers.^[1]

Cyclotomic fields played a crucial role in the development of modern algebra and number theory because of their relation with Fermat's Last Theorem. It was in the process of his deep investigations of the arithmetic of these fields (for prime n) – and more precisely, because of the failure of unique factorization in their rings of integers – that Ernst Kummer first introduced the concept of an ideal number and proved his celebrated congruences.