The differential operator $ D_{-} $ discretizes the
first derivative with the first-order formula
$$ \frac{\partial u_{i}}{\partial x} \approx
\frac{u_{i}-u_{i-1}}{h} = D_{-} u_{i}
$$
solve linear system for a given right-hand side
without result Array allocation. The rhs and result parameters
can be the same Array, in which case rhs will be changed
$ D_{-} $ matricial representation
The differential operator $ D_{-} $ discretizes the first derivative with the first-order formula $$ \frac{\partial u_{i}}{\partial x} \approx \frac{u_{i}-u_{i-1}}{h} = D_{-} u_{i} $$