1. Über Approximative Nomographie, I and II;Aumann;Bayer. Akad. Wiss. Math. - Nat.Kl.S.B.,1958
2. Ibid. (1959), 103–109.
3. Cheney, E. W. and W. A. Light, On the approximation of a bivariate function by the sum of univariate functions, Report CNA-140, Center for Numerical Analysis, University of Texas at Austin, August 1978, to appear, J. Approximation Theory.