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You can specify complex geometries by overlapping solid objects. This approach to representing geometries is called Constructive Solid Geometry (CSG).

Use these four solid objects to specify a geometry for your problem:

**Circle**— Represents the set of points inside and on a circle.**Polygon**— Represents the set of points inside and on a polygon given by a set of line segments.**Rectangle**— Represents the set of points inside and on a rectangle.**Ellipse**— Represents the set of points inside and on an ellipse. The ellipse can be rotated.

When you draw a solid object in the PDE app, each solid object is automatically given a unique name. Default names are C1, C2, C3, etc., for circles; P1, P2, P3, etc. for polygons; R1, R2, R3, etc., for rectangles; E1, E2, E3, etc., for ellipses. Squares, although a special case of rectangles, are named SQ1, SQ2, SQ3, etc. The name is displayed on the solid object itself. You can use any unique name, as long as it contains no blanks. In draw mode, you can alter the names and the geometries of the objects by double-clicking them, which opens a dialog box. The following figure shows an object dialog box for a circle.

You can use the name of the object to refer to the corresponding
set of points in a set formula. The operators +, *, and – are
used to form the set of points Ω in the plane over which the
differential equation is solved. The operators +, the set union operator,
and *, the set intersection operator, have the same precedence. The
operator –, the set difference operator, has higher precedence.
The precedence can be controlled by using parentheses. The resulting
geometrical model, Ω, is the set of points for which the set
formula evaluates to true. By default, it is the union of all solid
objects. We often refer to the area Ω as the *decomposed
geometry.*

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