ABSTRACT
We now consider triads (X, X 1, X 2), ordered triples of spaces such that X 1 and X 2 are subspaces of X. We have inclusion maps k 1 : ( X 2 , X 1 ∩ X 2 ) → ( X 1 ∪ X 2 , X 1 ) , k 2 : ( X 1 , X 1 ∩ X 2 ) → ( X 1 ∪ X 2 , X 2 ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502408/295d3acc-dfa3-4421-a2d3-a9c22ffb681c/content/eq411.tif"/>
