Problem Descriptor Reference > Introduction > Problem Descriptor Structure

Problem descriptors organize a problem by breaking it into sections of related items.

Each section is headed by a proper name followed by one or more statements which define the problem.

The permitted section names are:

TITLE	- defines the problem title
SELECT	- sets various options and controls
COORDINATES	- defines the coordinate system
VARIABLES	- names the problem variables
DEFINITIONS	- defines ancillary quantities and parameters
MATERIALS	- defines sets of parameters that can be applied by name
BOUNDARY CONDITIONS	- defines sets of boundary conditions that can be applied by name
INITIAL VALUES	- sets initial values of variables
EQUATIONS	- defines the partial differential equation system
CONSTRAINTS	- defines optional integral constraints
EXTRUSION	- extends the domain to three dimensions
BOUNDARIES	- describes the 2D or projected 3D domain
RESOLVE	- optionally supplements mesh refinement control
FRONT	- optionally supplements mesh refinement control for advancing fronts
TIME	- defines the time domain
MONITORS	- selects interim graphic display
PLOTS	- selects final graphic display
HISTORIES	- selects time-summary displays
END	- identifies the end of the descriptor

The number of sections used in a particular problem descriptor can vary, subject only to the requirement that all files must contain a BOUNDARIES section and an END section.

While some flexibility exists in the placement of these sections, it is suggested that the user adhere to the ordering described above.

DEFINITIONS and SELECT can appear more than once.

Because descriptors are dynamically processed from top to bottom, they cannot contain forward references. Definitions may refer to variables and other defined names, provided these variables and names have been defined in a preceding section or previously in the same section.

For detailed description of these sections, see "The Sections of a Descriptor".