Continuity in the Lagrangian description is expressed by the spatial and temporal continuity of the mapping from the reference configuration to the current configuration of the material points. All physical quantities characterizing the continuum are described this way. In this sense, the function χ χ ( ⋅ ⋅ ) and P i j … … ( ⋅ ⋅ ) are single-valued and continuous, with continuous derivatives with respect to space and time to whatever order is required, usually to the second or third.