1. Alexandrov B., Grantcharov G., Ivanov S.: Curvature properties of twistor spaces of quaternionic Kähler manifolds. J. Geom. 62(1–2), 1–12 (1998). doi: 10.1007/BF01237595
2. Alekseevsky D.V., Medori C., Tomassini A.: Homogeneous para-Kähler Einstein manifolds. Russ. Math. Surv. 64, 1–43 (2009)
3. Baum, H.: Twistor and killing spinors in Lorentzian geometry. In: Global Analysis and Harmonic Analysis, (Marseille-Luminy, 1999), Sémin. Congr., vol. 4, pp. 35–52. Soc. Math. France, Paris (2000). (English, with English and French summaries.)
4. Besse, A.L.: Einstein manifolds. Ergebnisse der Mathematik und ihrer Grenzgebiete (3) [Results in Mathematics and Related Areas (3)], vol. 10. Springer-Verlag, Berlin (1987)
5. Blažić N., Vukmirović S.: Examples of self-dual, Einstein metrics of (2, 2)- signature. Math. Scand. 94(1), 63–74 (2004)