src/HOL/Algebra/abstract/Ideal.thy
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(*
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    Ideals for commutative rings
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    $Id$
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    Author: Clemens Ballarin, started 24 September 1999
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*)
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theory Ideal imports Ring begin
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consts
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  ideal_of      :: "('a::ring) set => 'a set"
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  is_ideal      :: "('a::ring) set => bool"
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  is_pideal     :: "('a::ring) set => bool"
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defs
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  is_ideal_def:  "is_ideal I == (ALL a: I. ALL b: I. a + b : I) &
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                                (ALL a: I. - a : I) & 0 : I &
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                                (ALL a: I. ALL r. a * r : I)"
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  ideal_of_def:  "ideal_of S == Inter {I. is_ideal I & S <= I}"
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  is_pideal_def: "is_pideal I == (EX a. I = ideal_of {a})"
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text {* Principle ideal domains *}
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axclass pid < "domain"
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  pid_ax: "is_ideal I ==> is_pideal I"
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end