closed set , open set

T

= { V( S ) | S ⊂ R }

と置くと

1 )

 φ , Spec( R ) ∈ T

2 )

∩ V( S ) ( 任意個 ) ∈ T

3 )

∪ V( S ) ( 有限個 ) ∈ T

を満たす・・・

⇒

T : topology of R

V( S ) : closed set of R

D( S ) : open set of R

という.