Talk:Excision theorem

[Untitled]
"The theorem states that if the closure of U is contained in the interior of A, then U can be excised". Is this the closure of U in X, rather than A? I think it should be but didn't want to change it without checking. This makes a real difference since A is always closed in itself. — Preceding unsigned comment added by 131.111.8.98 (talk) 09:44, January 9, 2007‎ (UTC)

Yes - the closure of U in X, and the interior of A in X (the interior of A in A is again A). — Preceding unsigned comment added by 132.66.222.96 (talk) 06:43, February 10, 2007‎ (UTC)

Intuition/Applications Needed
This page should give explanations as to how excision is actually useful. For example, it should describe how excision can be used for defining local cohomology, as is Hatcher page 126. — Preceding unsigned comment added by 161.98.8.4 (talk) 19:26, 9 August 2017 (UTC)