Brownfield sites are abandoned or underused industrial and commercial facilities available for re-use. Expansion or redevelopment of such a facility may be complicated by real or perceived environmental contaminations. In the United States city planning jargon, brownfield site (or simply a brownfield) is land previously used for industrial purposes or certain commercial uses.
In mathematics, a differential operator is an operator defined as a function of the differentiation operator. It is helpful, as a matter of notation first, to consider differentiation as an abstract operation, accepting a function and returning another (in the style of a higher-order function in computer science). There are certainly reasons not to restrict to linear operators; for instance the Schwarzian derivative is a well-known non-linear operator.