On entry: the point at which components of the solution and derivative are to be evaluated.
twant should not normally be an extrapolation point, that is
twant should satisfy
- $\mathit{told}\le {\mathbf{twant}}\le \mathrm{T}$,
or if integration is proceeding in the negative direction
- $\mathit{told}\ge {\mathbf{twant}}\ge \mathrm{T}$,
where
$\mathit{told}$ is the previous integration point and is, to within rounding,
tcurr –
hlast (see
d02qxf). Extrapolation is permitted but not recommended and
${\mathbf{ifail}}={\mathbf{2}}$ is returned whenever extrapolation is attempted.