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In the mathematical field of set theory, an ideal is a collection of sets that are considered to be "small" or "negligible". Every subset of an element of the ideal must also be in the ideal (this codifies the idea that an ideal is a notion of smallness), and the union of any two elements of the ideal must also be in the ideal. More formally, given a set ''X'', an ideal ''I'' on ''X'' is a nonempty subset of the powerset of ''X'', such that: # if ''A'' ∈ ''I'' and ''B'' ⊆ ''A'', then ''B'' ∈ ''I'', and # if ''A'',''B'' ∈ ''I'', then ''A''∪''B'' ∈ ''I''. Some authors add a third condition that ''X'' itself is not in ''I''; ideals with this extra property are called proper ideals. Ideals in the set-theoretic sense are exactly ideals in the order-theoretic sense, where the relevant order is set inclusion. Also, they are exactly ideals in the ring-theoretic sense on the Boolean ring formed by the powerset of the underlying set. ==Terminology== An element of an ideal ''I'' is said to be ''I-null'' or ''I-negligible'', or simply ''null'' or ''negligible'' if the ideal ''I'' is understood from context. If ''I'' is an ideal on ''X'', then a subset of ''X'' is said to be ''I-positive'' (or just ''positive'') if it is ''not'' an element of ''I''. The collection of all ''I''-positive subsets of ''X'' is denoted ''I''+. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ideal (set theory)」の詳細全文を読む スポンサード リンク
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