This function takes a vector of appropriate length (typically created using
vech) and creates a symmetric matrix.
Usage
xpnd(x, nrow = NULL)
Value
An \((nrows \times nrows)\) symmetric matrix.
Arguments
x
A list of elements to expand into symmetric matrix.
nrow
The number of rows (and columns) in the returned matrix. Look
into the details.
Details
This function is particularly useful when dealing with variance covariance
matrices. Note that R stores matrices in column major order, and that the
items in x will be recycled to fill the matrix if need be.
The number of rows can be specified or automatically computed from the
number of elements in a given object via \((-1 + \sqrt{(1 + 8 *
length(x))}) / 2\).