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A new class of R^2 type of actions without ghosts and tachyons is found in which both the vierbein field {e_}^a and the spin connection {_}^ab are independent and propagating fields. A complete set of spin-projection operators for {e_}^a and {_}^ab is constructed. The particle content of these actions and other R^2 theories in the literature is given. The relation of this work to the interesting work of Neville is discussed.
Sezgin et al. (Sun,) studied this question.