1. Bergmann, P. G. (1968).International Journal of Theoretical Physics, Vol. 1, No. 1, p. 25.
2. Brans, C. and Dicke, R. H. (1961).Physical Review,124, 925.
3. du Plessis, J. C. (1969).Tensor,20, 347.
4. Horndeski, G. W. (1971). ‘Mathematical Aspects of Scalar-Tensor Field Theories’, Section 2, M. Math. thesis (unpublished), University of Waterloo.
5. Horndeski, G. W. and Lovelock, D. (1972).Tensor,24, 79.