Sigmoidal logistic function, type 1.
Reference: Seber, G. A. F., Wild, C. J. 1989. Nonlinear Regression. John Wiley & Sons, Inc. pp. 328 - 330
Number: 3
Names: a, xc, k
Meanings: a = amplitude, xc = center, k = coefficient
Lower Bounds: xc > 0
Upper Bounds: none
nlf_slogistic1(x,a,xc,k)
FITFUNC\SLOGIST1.FDF
Growth/Sigmoidal