1. Melvin S. Berger, Nonlinearity and Functional Analysis: Lectures on Nonlinear Problems in Mathematical Analysis, Pure Appl. Math. 74, Academic Press, New York, 1977.
2. Arthur L. Besse, Einstein Manifolds, Ergeb. Math. Grenzgeb. (3) 10, Springer-Verlag, Berlin, 1987.
3. Robert Bryant, Phillip A. Griffiths, and Lucas Hsu, The Poincaré-Cartan form, Institute for Advanced Study preprint, 1997.
4. L. Caffarelli, L. Nirenberg, and J. Spruck, The Dirichlet problem for nonlinear second-order elliptic equations, III: Functions of the eigenvalues of the Hessian, Acta Math. 155 (1985), 261--301.
5. Élie Cartan, Les espaces à connexion conforme, Ann. Soc. Polon. Math. 2 (1923), 171--221.