**Is the following presumed folklore fact written anywhere?**

*Let $E^*$ a multiplicative cohomology theory. Then the coboundary map in the Mayer–Vietoris sequence of an excisive triad $(X;U,V)$ preserves the $E^*X$-module structures induced by the inclusions.*

The proof follows from two other statements that also seem not to show up in introductory texts.

The first appears in a MathOverflow answer but nowhere else I'm aware of:

*The Mayer–Vietoris sequence for a pointed excisive triad $(X;U,V)$ arises as the long exact sequence of the cofibration $U \vee V \to U \cup \big((U \cap V) \wedge I\big) \cup V \simeq X.$*

And this one is stated as Proposition 2.15 of Allen Hatcher's *Vector Bundles and K-Theory* (for complex topological K-theory):

*Let $E^*$ be a multiplicative cohomology theory. Every object in the long exact sequence of a pair $(X,A)$ is a module over $E^*X$ in such a way that all maps are $E^*X$-module homomorphisms.*

The proof uses only that the external product is natural, the smash-diagonal $X \to X \wedge X$ makes $X$ into a coalgebra, and that allied maps make the Puppe sequence of $A \hookrightarrow X$ into a sequence of $X$-comodules, so it should hold in general.

**Are the forms of these I want written anywhere?**

All of this condenses an unanswered Math.StackExchange question.