1. Leviatan, D., Degree of Copositive Approximation, In: Approximation Theory IV, Academic Press, New York, 1983, 587–592.
2. Passow, E. and Raymon, L., Copositive Polynomial Approximation, J. Approx. Theory 12(1974), 299–304.
3. Roulier, J.A., The Degree of Copositive Approximation, J. Approx. Theory 19(1977), 253–258.
4. Telyakovskii, S. A., Research in the Theory of Approximation of Functions at the Mathematical Institute of the Academy of Sciences, Proc. Steklov Institute Math. 182(1988), 141–197.
5. Yu, X. M., Degree of Copositive Approximation, Chinese Ann. Math. Ser. B 10(1989), 409–415. MR 91b:41016.