A
simple polygon
In geometry, a simple polygon is a polygon that does not intersect itself and has no holes. That is, it is a flat shape consisting of straight, non-intersecting line segments or "sides" that are joined pairwise to form a single closed path. If ...
that is not
convex
Convex or convexity may refer to:
Science and technology
* Convex lens, in optics
Mathematics
* Convex set, containing the whole line segment that joins points
** Convex polygon, a polygon which encloses a convex set of points
** Convex polytop ...
is called concave, non-convex or reentrant. A concave polygon will always have at least one
reflex interior angle—that is, an angle with a measure that is between 180 degrees and 360 degrees exclusive.
Polygon
Some lines containing interior points of a concave polygon intersect its boundary at more than two points.
[ Some ]diagonals
In geometry, a diagonal is a line segment joining two vertices of a polygon or polyhedron, when those vertices are not on the same edge. Informally, any sloping line is called diagonal. The word ''diagonal'' derives from the ancient Greek δ ...
of a concave polygon lie partly or wholly outside the polygon.[ Some ]sidelines
The "sidelines" are the white or colored lines which mark the outer boundaries of a sports field, running parallel to each other and perpendicular to the goal lines. The sidelines are also where the coaching staff and players out of play ope ...
of a concave polygon fail to divide the plane into two half-planes one of which entirely contains the polygon. None of these three statements holds for a convex polygon.
As with any simple polygon, the sum of the internal angle
In geometry, an angle of a polygon is formed by two sides of the polygon that share an endpoint. For a simple (non-self-intersecting) polygon, regardless of whether it is convex or non-convex, this angle is called an interior angle (or ) if ...
s of a concave polygon is ×(''n'' − 2) radian
The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before that ...
s, equivalently 180×(''n'' − 2) degrees (°), where ''n'' is the number of sides.
It is always possible to partition a concave polygon into a set of convex polygons. A polynomial-time algorithm for finding a decomposition into as few convex polygons as possible is described by .[.]
A triangle
A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC.
In Euclidean geometry, any three points, when non- colline ...
can never be concave, but there exist concave polygons with ''n'' sides for any ''n'' > 3. An example of a concave quadrilateral
In geometry a quadrilateral is a four-sided polygon, having four edges (sides) and four corners (vertices). The word is derived from the Latin words ''quadri'', a variant of four, and ''latus'', meaning "side". It is also called a tetragon, ...
is the dart
Dart or DART may refer to:
* Dart, the equipment in the game of darts
Arts, entertainment and media
* Dart (comics), an Image Comics superhero
* Dart, a character from ''G.I. Joe''
* Dart, a ''Thomas & Friends'' railway engine character
* Da ...
.
At least one interior angle does not contain all other vertices in its edges and interior.
The convex hull
In geometry, the convex hull or convex envelope or convex closure of a shape is the smallest convex set that contains it. The convex hull may be defined either as the intersection of all convex sets containing a given subset of a Euclidean space ...
of the concave polygon's vertices, and that of its edges, contains points that are exterior to the polygon.
Notes
External links
*{{mathworld , urlname=ConcavePolygon , title=Concave polygon
Types of polygons