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78(1): 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE [sage] 2018/01/01(月) 17:14:45.77 ID:dCRrvhl7(14/27) AAS
>>77 つづき

https://en.wikipedia.org/wiki/Sigma-ideal
σ-ideal Sigma-ideal (Redirected from Σ-ideal)
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In mathematics, particularly measure theory, a σ-ideal of a sigma-algebra (σ, read "sigma," means countable in this context) is a subset with certain desirable closure properties. It is a special type of ideal. Its most frequent application is perhaps in probability theory.

Let (X,Σ) be a measurable space (meaning Σ is a σ-algebra of subsets of X). A subset N of Σ is a σ-ideal if the following properties are satisfied:

(i) O ∈ N;
(ii) When A ∈ N and B ∈ Σ , B ⊆ A ⇒ B ∈ N;
(iii) {A_n}_{n∈N }⊆ N→ ∪ _{n∈N }A_n∈ N.

Briefly, a sigma-ideal must contain the empty set and contain subsets and countable unions of its elements. The concept of σ-ideal is dual to that of a countably complete (σ-) filter.

If a measure μ is given on (X,Σ), the set of μ-negligible sets (S ∈ Σ such that μ(S) = 0) is a σ-ideal.

The notion can be generalized to preorders (P,?,0) with a bottom element 0 as follows: I is a σ-ideal of P just when

(i') 0 ∈ I,
(ii') x ? y & y ∈ I ⇒ x ∈ I, and
(iii') given a family xn ∈ I (n ∈ N), there is y ∈ I such that xn ? y for each n

Thus I contains the bottom element, is downward closed, and is closed under countable suprema (which must exist). It is natural in this context to ask that P itself have countable suprema.

A σ-ideal of a set X is a σ-ideal of the power set of X. That is, when no σ-algebra is specified, then one simply takes the full power set of the underlying set. For example, the meager subsets of a topological space are those in the σ-ideal generated by the collection of closed subsets with empty interior.
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79(1): 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE [sage] 2018/01/01(月) 17:15:30.82 ID:dCRrvhl7(15/27) AAS
>>78 つづき

https://en.wikipedia.org/wiki/Ideal
Ideal
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Mathematics
Ideal (ring theory), special subsets of a ring considered in abstract algebra
Ideal, special subsets of a semigroup
Ideal (order theory), special kind of lower sets of an order
Ideal (set theory), a collection of sets regarded as "small" or "negligible"
Ideal (Lie algebra), a particular subset in a Lie algebra
Ideal point, a boundary point in hyperbolic geometry
Ideal triangle, a triangle in hyperbolic geometry whose vertices are ideal points
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