One of the Eilenberg-Steenrod axioms which states that, if 
is a space with subspaces 
and 
such that the set closure of
is contained in the interior of 
, then the inclusion
map 
induces an isomorphism 
.
Excision Axiom
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Cite this as:
Weisstein, Eric W. "Excision Axiom." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ExcisionAxiom.html