1. A. S. Eddington, The mathematical theory of relativity, 2nd ed., Cambridge (1954).
2. A. Einstein, Spielen Gravitationsfelder im Aufbau der materiellen Elementarteilchen eine wesentliche Rolle? Sitzber. Preuß. Akad. Wiss.1919, 349–355 (1919).
3. C. W. Misner andJ. A. Wheeler, Classical Physics as Geometry. Annals Phys.2, 525–603 (1957).
4. G. Y. Rainich, Electrodynamics in the General Relativity Theory. Trans. Amer. Math. Soc.27, 106–130 (1925).
5. H. Rund, Variational problems in which the unknown functions are tensor components. Second Colloquium on the Calculus of Variations, 129–174, University of South Africa (1964).