Hello All,
I need some help with optimx, mostly due to my apparent lack of imagination
than optimx itself. Below is the pertinent code for which I have a question.
I am fitting to the term structure of swap rates per Cox, Ingersoll, and Ross.
As you can see the objective function is CIRTune. I have set up the lower and
upper bounds of the model for each kappa, lambda, and theta. Everything works
as expected.
However, there is an additional constraint.
2 * kappa * lamba >= sigma^2
My question is as follows: should the constraint be worked into the function
or should I add an additional parameter in the function that is controlled by
lower and upper?
#Objective function
CIRTune <- function(param = numeric(), shortrate = numeric(), sigma = .015,
cfmatrix = matrix(), matmatrix = matrix()){
kappa = param[1]
lambda = param[2]
theta = param[3]
Disc <- CIRBondPrice(kappa = kappa, lambda = lambda, theta = theta,
shortrate = shortrate, T= matmatrix, step = 0, sigma = sigma)
CIRTune <- sqrt((sum(colSums((cfmatrix * Disc))^2))/ncol(matmatrix))
return(CIRTune)
}
# Fit the model to the market
fit <- optimx(par = c(.1, .003, .03),
fn = CIRTune,
method = "L-BFGS-B",
lower = c(rep(.001,3),
upper = rep(1, 3),
shortrate = shortrate,
sigma = .015,
cfmatrix = CIR.CF.Matrix,
matmatrix = CIR.Mat.Matrix)
close(CalCIR1)
return(fit)
Thank-you in advance,
Glenn
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