Ultraspherical rational filters
David E. Johnson, J. Robert Johnson, M. Kashefi · IEEE Transactions on Circuit Theory · 1973
A set of rational functions based on ultraspherical polynomials is developed and used to approximate the ideal amplitude response of a low-pass filter, including by conjecture the elliptic filter as a special case. The procedure described for obtaining the elliptic filter avoids the necessity of computing the modulus of the elliptic functions, and requires no knowledge of elliptic function theory.