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Removing Ostrogradski's ghost from cosmological perturbations in $f(R,R_{μν}^2,C_{μνρσ}^2)$ gravity

Recently it was argued that gravity with the squire of the Ricci tensor can be stabilized by adding constraints to the theory. This was so far demonstrated for fluctuations on the Minkowski/de Sitter background. We show that the same scheme works equally well for removing Ostrogradski's ghost from fluctuations on a cosmological background in generic $f(R,R_{μν}^2,C_{μνρσ}^2)$-type theories of gravity. We also derive the general formula for the spectrum of primordial tensor perturbations from the stabilized theory.