A Hecke character (or Hecke character of the second kind) is a particular type of character associated with algebraic number fields and arithmetic functions. More specifically, these characters arise in the study of modular forms and algebraic K-theory.
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A Hecke character of a number field is a continuous character of its idèle class group. Its finite conductor records the local unit subgroups on which it is trivial. As an ideal character it is defined on fractional ideals prime to that conductor, with a prescribed rule on principal ideals coming from its archimedean character. A CM elliptic curve yields an algebraic Hecke character with one CM embedding in its infinity type of a Hecke character.