Cyclotomic fields and abelian Galois groups
Research context
postgraduate concept · established · As of 2026-10-10
For a primitive n-th root zeta-n, Gal(Q(zeta-n)/Q) is isomorphic to the units modulo n by a↦(zeta-n↦zeta-n^a), and the degree is Euler phi(n).
Key knowledge
For a primitive n-th root zeta-n, Gal(Q(zeta-n)/Q) is isomorphic to the units modulo n by a↦(zeta-n↦zeta-n^a), and the degree is Euler phi(n). For n=5 the group is cyclic of order four; for n=8 it is C2×C2. Cyclotomic polynomials are irreducible over Q; a different base field can have a smaller group.
Values and equations
[Q(zeta_n):Q]=phi(n); Gal(Q(zeta_n)/Q) ≅ (Z/nZ)^×
- zeta_n
- Primitive n-th complex root of unity
- phi(n)
- Number of units modulo n
Conditions and scope
- n is a positive integer and the base field in the stated full-group result is Q.
- In positive characteristic, separation of x^n−1 requires that the characteristic not divide n.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: Examples 2.6/2.10 p.29: cubic splitting fields and roots; Example 3.8 p.37: x-cubed minus2 nonnormal real field; Example 4.7 p.49: cubic A3/S3 criterion, applied in the recorded worked example; cyclotomic extensions pp.64–66; solvability Theorem 5.34 pp.76–80. The check is bounded to the recorded claims; no complete proof reconstruction, software proof replay or universal literature completeness is claimed.
2026-10-10 · Author source-body/statement/equation/domain check, with graph associations scoped individually. Separate independent content/source/inventory review is required before branch closure.
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