1955, Frank S. Ham, The Quantum Defect Method, Frederick Seitz, David Turnbull (editors), Solid State Physics, Volume 1, Academic Press, page 138,
From thisCoulombfunction, we can thencalculate the logarithmic derivativeof at the eigenvalue for any value of outside the core. If there exists a radius lying in the Coulomb region and sufficiently small that the logarithmic derivative at is a smooth function of the energy, we can obtain the logarithmic derivative for arbitrary energies within a reasonable range by interpolating it between the eigenvalues.
1994, Bjarne S. Jensen, The Dynamic Systems of Basic Economic Growth Models, Kluwer Academic, page 329,