Hi,

My problem is that i will use rbpspline{SpatialExtremes} to fit a penalized
spline with radial basis functions to my data
but i have two predictor variables and i cannot select knots.
In R help there is an example such that



n <- 200
x <- runif(n)
fun <- function(x) sin(3 * pi * x)
y <- fun(x) + rnorm(n, 0, sqrt(0.4))
knots <- quantile(x, prob = 1:(n/4) / (n/4 + 1))
fitted <- rbpspline(y, x, knots = knots, degree = 3)
fitted
plot(x, y)
lines(fitted, col = 2)

In my case,

n<-200
x1<-runif(n)
x2<-runif(n)
fun<-function(x1,x2) 2*x1+x2^2
y <- fun(x1,x2) + rnorm(n, 0, sqrt(0.4))

 but at this point i do not know how to select knots.
I want to take cross products of quantiles of x1 and x2.
In R help it says that knots is defined as a vector that gives the
coordinates of the knots.
How can i give the coordinates of two dimensional knots as a vector?



Thanks in advance

	[[alternative HTML version deleted]]

