Extended gravity theories and the Einstein--Hilbert action

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Abstract
I discuss the relation between arbitrarily high-order theories of gravity and scalar--tensor gravity at the level of the field equations and the action. I show that (2n+4)-order gravity is dynamically equivalent to Brans--Dicke gravity with an interaction potential for the Brans--Dicke field and n further scalar fields. This scalar--tensor action is then conformally equivalent to the Einstein--Hilbert action with n+1 scalar fields. This clarifies the nature and extent of the conformal equivalence between extended gravity theories and general relativity with many scalar fields.
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