Piecewise nls w/ boundary as a fitting parameter

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Piecewise nls w/ boundary as a fitting parameter

 Hello, Fitting a piecewise smooth curve to a set of points (and a piecewise linear function in particular) seems to be a recurring question on this list. Nevertheless, I was not able to find an answer to a question that bothers me. Suppose I have the following data set, and would want to fit it with a piecewise smooth curve, In this model data, one curve is valid for up to 3 and another one for onwards. However, suppose I don't have this data. x <- 0:8 y <- c(0.5, 0.5, 0.5, 0.5, 1.914214, 2.5, 2.949490, 3.328427, 3.662278) plot(x, y) # Neither of these seem to work f <- function(x, x0, y0, k) { ifelse(x < x0, y0, y0 + sqrt(k*(x-x0))) } # f <- function(x, x0, y0, k) { (x < x0)*y0 + (x >= x0)*(y0 + sqrt(k*(x-x0))) } fmodel <- nls(y ~ f(x, x0, y0, k), start = list(x0 = 1, y0 = 1, k = 1)) What's the proper way to deal with such a case? Am I supposed to somehow obtain the break point separately, and then fit only y0 and k? What is the preferred method for obtaining break point (I am not even sure if I am using this term correctly). Thanks in advance, Primož -- Primož Peterlin,   Inštitut za biofiziko, Med. fakulteta, Univerza v Ljubljani Lipičeva 2, SI-1000 Ljubljana, Slovenija.        [hidden email] Tel +386-1-5437612, fax +386-1-4315127,  http://biofiz.mf.uni-lj.si/~peterlin/F8021D69 OpenPGP fingerprint: CB 6F F1 EE D9 67 E0 2F  0B 59 AF 0D 79 56 19 0F ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.