Sigmoidal Richards function, type 1
Reference: Seber, G. A. F., Wild, C. J. 1989. Nonlinear Regression. John Wiley & Sons, Inc. pp. 332 - 337
Number: 4
Names: a, xc, d, k
Meanings: a = top asymptote, xc = center, d = unknown, k = coefficient
Lower Bounds: a > 0.0, k > 0.0
Upper Bounds: none
nlf_srichards1(x,a,xc,d,k)
FITFUNC\SRICHAR1.FDF
Growth/Sigmoidal