make.del takes a parameter vector of length \(k*(k+1)/2\) and
returns the upper triangular \(k \times k\) matrix \(\Delta\).
make.del is a private function intended for use inside mlest.
make.del(pars)A length \(k*(k+1)/2\) numerical vector giving the elements of \(\Delta\).
An upper triangular \(k \times k\) matrix.
The first \(k\) elements of pars are the log of the diagonal
elements of \(\Delta\). The next \(k*(k-1)/2\) elements are the
elements above the main diagonal of \(\Delta\), ordered by column
(left to right), and then by row within column (top to bottom). That
is to say, if \(\Delta_{ij}\) is the element in the \(i\)th row
and \(j\)th column of \(\Delta\), then the order of the parameters
is \(\Delta_{11}, \Delta_{22}, \ldots, \Delta_{kk}, \Delta_{12},
\Delta_{13}, \Delta_{23}, \Delta_{14}, \ldots,\Delta_{(k-1)k}\).
Pinheiro, J. C., and Bates, D. M. (2000) Mixed-effects models in S and S-PLUS. New York: Springer.