Dear Prof. Murdoch and Dr. Winsemius,

Thank you very much for your suggestion. They were big help.
One of my friends told me another approach by using "smooth.spline". Here is
the code:

x <- c(1,2,3,4,5,6)
y <- c(1,4,9,16,25,36)
sp <- smooth.spline(x,y)
x <- seq(1, 6, length=100)
pred <- predict(sp, x)
real <- x^2
err <- real - pred$y
cbind(pred$x, pred$y, real, err)
plot(pred)

Thank a lot. Have a nice weekend  :)

Wang Zhipeng

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