In algebraic number theory, a reflection theorem or Spiegelungssatz (German for reflection theorem – see Spiegel and Satz) is one of a collection of theorems linking the sizes of different ideal class groups (or ray class groups), or the sizes of different isotypic components of a class group. The original example is due to Ernst Eduard Kummer, who showed that the class number of the cyclotomic field Q ( ζ p ) {\displaystyle \mathbb {Q} \left(\zeta _{p}\right)} , with p a prime number, will be divisible by p if the class number of the maximal real subfield Q ( ζ p ) + {\displaystyle \mathbb {Q} \left(\zeta _{p}\right)^{+}} is. Another example is due to Scholz. A simplified version of his theorem states that if 3 divides the class number of a real quadratic field Q ( d ) {\displaystyle \mathbb {Q} \left({\sqrt {d}}\right)} , then 3 also divides the class number of the imaginary quadratic field Q ( − 3 d ) {\displaystyle \mathbb {Q} \left({\sqrt {-3d}}\right)} .
Leopoldt's Spiegelungssatz Both of the above results are generalized by Leopoldt's "Spiegelungssatz", which relates the p-ranks of different isotypic components of the class group of a number field considered as a module over the Galois group of a Galois extension. Let L/K be a finite Galois extension of number fields, with group G, degree prime to p and L containing the p-th roots of unity. Let A be the p-Sylow subgroup of the class group of L. Let φ run over the irreducible characters of the group ring Qp[G] and let Aφ denote the corresponding direct summands of A. For any φ let q = pφ(1) and let the G-rank eφ be the exponent in the index
[ A ϕ : A ϕ p ] = q e ϕ . {\displaystyle [A_{\phi }:A_{\phi }^{p}]=q^{e_{\phi }}.}
Let ω be the character of G
ζ g = ζ ω ( g ) for ζ ∈ μ p . {\displaystyle \zeta ^{g}=\zeta ^{\omega (g)}{\text{ for }}\zeta \in \mu _{p}.}
The reflection (Spiegelung) φ* is defined by
ϕ ∗ ( g ) = ω ( g ) ϕ ( g − 1 ) . {\displaystyle \phi ^{*}(g)=\omega (g)\phi (g^{-1}).}
Let E be the unit group of K. We say that ε is "primary" if K ( ϵ p ) / K {\displaystyle K({\sqrt[{p}]{\epsilon }})/K} is unramified, and let E0 denote the group of primary units modulo Ep. Let δφ denote the G-rank of the φ component of E0. The Spiegelungssatz states that
| e ϕ ∗ − e ϕ | ≤ δ ϕ . {\displaystyle |e_{\phi ^{*}}-e_{\phi }|\leq \delta _{\phi }.}
Extensions Extensions of this Spiegelungssatz were given by Oriat and Oriat-Satge, where class groups were no longer associated with characters of the Galois group of K/k, but rather by ideals in a group ring over the Galois group of K/k. Leopoldt's Spiegelungssatz was generalized in a different direction by Kuroda, who extended it to a statement about ray class groups. This was further developed into the very general "T-S reflection theorem" of Georges Gras. Kenkichi Iwasawa also provided an Iwasawa-theoretic reflection theorem.
References
Koch, Helmut (1997). Algebraic Number Theory. Encycl. Math. Sci. Vol. 62 (2nd printing of 1st ed.). Springer-Verlag. pp. 147–149. ISBN 3-540-63003-1. Zbl 0819.11044.
