From the definition of the Erdös number, to establish a bound of 4 for mine, it suffices to supply the following citations:

  1. Anderson, J.M., Erdös, P., Pinkus, A, Shisha, Oved,
    The closed linear span of \(\{x^k-c_k\}_1^{\infty}\)
    J. Approx. Theory 43 (1985), pp. 75--80.
  2. Pinkus, A, Braess, D,
    Interpolation by Ridge Functions,
    J. Approx. Theory 73 (1993), pp. 218-236
  3. Braess, D, C. Schwab,
    Approximation on simplices with respect to weighted Sobolev norms.
    J. Approx. Theory 103 (2001), No. 2, 329-337.
  4. J.M. Melenk, C. Schwab,
    Analytic regularity for a singularly perturbed problem,
    SIAM J. Math. Anal. 30 (1999), 379-400