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Published online by Cambridge University Press: 24 October 2008
The object of this paper is to generalize the relative Mayer-Vietoris sequence of Eilenberg and Steenrod([3], p. 42) by allowing both the containing spaces and the subspaces to vary. Fix a generalized homology theory h*. We call a couple of spaces {X1, X2} excisive (for h*) if they are subspaces of some larger space and the inclusion (X1, X1 ∩ X2) → (X1 ∪ X2, X2) induces an isomorphism under h* (see [4], pp. 208–9 for equivalent definitions).