1. Borunda , M. Janssen , B. Bastero-Gil , M. 2008 Palatini versus Metric Formulation in Higher-Curvature Gravity, arXiv:0804.4440 [hep-th]
2. De Donder , T. 1930 Théorie Invariantive du Calcul des Variations, (Gaulthier-Villars & Cie., Paris)
3. Einstein , A. 1918 Personal letter to H. Weyl, ETH Zürich Library, Archives and Estates
4. Misner , Ch.W. Thorne , K.S. Wheeler , J.A. 1973 Gravitation, (W.H. Freeman and Company, New York)
5. Quadratic lagrangians and general relativity