1. Y. Agaoka, On the curvature of Riemannian submanifolds of codimension 2. Hokkaido Math. J. 14, 107135 (1985)
2. D. Burns, Twistors and harmonic maps, in Lecture, Amer. Math. Soc. Conference (Char lotte, NC, 1986)
3. E. Cartan, Sur la possibilite de plonger un espace Riemannien donne dans espace Euclidean. Ann. Soc. Polon. Math. 6 1–7 (1927)
4. M. Dunajski, S.A. Hartnoll, Einstein-Maxwell gravitational instantons and five dimensional solitonic strings. Class. Quantum. Grav. 24, 1841–1862 (2007). hep-th/0610261
5. M. Dunajski, in Solitons, Instantons, and Twistors. Oxford Graduate Texts in Mathematics (Oxford University Press, Oxford, 2009)