Abstract
It is shown that the solutions of the field equations generated by the most general effective quadratic LagrangianaR2+bRklRklwhich are static, singularity-free, of classC4and asymptotically flat (of rankA3) are (i) conformally flat when 3a+b= 0, (ii) flat when 3a+b≠ 0 (b≠ 0), and (iii) have zero scalar curvature whenb= 0.

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