Please see:

<UL><LI>
S. A. Fulling, R. C. King, B. G. Wybourne, and C. J. Cummins,
Normal forms for tensor polynomials. I,
Classical and Quantum Gravity 9 (1992), 1151-1198.

<LI>
S. A. Fulling, C. J. Cummins, R. C. King, and B. G. Wybourne, 
<a href="./goslar.pdf">Group-theoretical classification of polynomial 
functions of the Riemann tensor</a>,
in Classical and Quantum Systems -- Foundations and Symmetries,
ed. by H. D. Doebner, W. Scherer, and F. Schroeck
[proceedings of the Second International Wigner Symposium, 
Goslar, Germany, 1991], 
World Scientific, Singapore, 1993, pp. 362-365.

</ul>