The statements of class field theory

Chapter 2 The statements of class field theory

We next give the statements of the principal results of class field theory, with almost no proofs. Our goal at this point is to clarify what the statements say and how they can be applied. We will have to discuss plenty of other material before returning to the proofs, but the reader who wishes to peek ahead for a glimpse of the strategy is directed to Section 5.4.

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Definition 2.0.1. Jargon watch.

By a place of a number field

K,

we mean either an archimedean completion

K ↪ R

K ↪ C

(an infinite place), or a

p

-adic completion

K ↪ K_{p}

for some nonzero prime ideal

p

o_{K}

(a finite place).

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Each place corresponds to an equivalence class of absolute values on

K;

v

is a place, we write

K_{v}

for the corresponding completion, which is either

R,

C,

K_{p}

for some prime

p .

(This means that two complex embeddings of

K

which differ by complex conjugation are considered the same place of

K .

)

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This form of parity between finite and infinite places will be a recurring theme throughout this book.

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