On exit:
QXji contains the value of the partial derivatives with respect to
xj of the interpolant
Q x at
xi, for
i=1,2,…,n, and for each of the four partial derivatives
j=1,2,3,4. If any of these evaluation points lie outside the region of definition of the interpolant, the corresponding entries in
QX are set to the largest machine representable number (see
X02ALF), and E01TLF returns with
IFAIL=3.