symmetry
Let V be a Euclidean vector space, F⊆V, and E:V→V be a Euclidean transformation that is not the identity map.
The following terms are used to indicate that E(F)=F if E is a rotation:
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F has rotational symmetry
;
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F has point symmetry;
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F has symmetry about a point;
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F is symmetric about a point.
If V=ℝ2, then the last two terms may be used to indicate the specific case in which E is conjugate to (-100-1), i.e. (http://planetmath.org/Ie) the angle of rotation is 180∘.
The following are classic examples of rotational symmetry in ℝ2:
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Regular polygons
: A regular
n-gon is symmetric about its center (http://planetmath.org/Center9) with valid angles of rotation θ=(360kn)∘ for any positive integer k<n.
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Circles: A circle is symmetric about its center (http://planetmath.org/Center8) with uncountably many valid angles of rotation.
As another example, let F=4⋃k=1Pk, where each Pk is defined thus:
P1 | = | {(x,y):0≤x≤41+√3 and (2-√3)x≤y≤x}, | ||
P2 | = | {(x,y):41+√3≤x≤2 and x≤y≤(2+√3)x-4}, | ||
P3 | = | {(x,y):2≤x≤4√31+√3 and (-2+√3)x+8-4√3≤y≤(-2-√3)x+4+4√3}, | ||
P4 | = | {(x,y):4√31+√3≤x≤4 and (-2+√3)x+8-4√3≤y≤-x+4}. |
Then F has point symmetry with respect to the point (2,2√3). The valid angles of rotation for F are 120∘ and 240∘. The boundary of F and the point (2,2√3) are shown in the following picture.
As a final example, the figure
{(x,y):-3≤x≤-1 and (x+1)2+y2≤4}∪([-1,1]×[-2,2])∪{(x,y):1≤x≤3 and (x-1)2+y2≤4} is symmetric about the origin. The boundary of this figure and the point (0,0) are shown in the following picture.
If E(F)=F and E is a reflection, then F has reflectional symmetry. In the special case that V=ℝ2, the following terms are used:
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F has line symmetry;
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F has symmetry about a line;
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F is symmetric about a line.
The following are classic examples of line symmetry in ℝ2:
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Regular polygons: There are n lines of symmetry
of a regular n-gon. Each of these pass through its center and at least one of its vertices.
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Circles: A circle is symmetric about any line passing through its center.
As another example, the isosceles trapezoid defined by
T={(x,y):0≤x≤6 and 0≤y≤min{x,2,-x+6}} |
is symmetric about x=3.
In the picture above, the boundary of T is drawn in black, and the line x=3 is drawn in cyan.
Title | symmetry |
Canonical name | Symmetry |
Date of creation | 2013-03-22 17:12:29 |
Last modified on | 2013-03-22 17:12:29 |
Owner | Wkbj79 (1863) |
Last modified by | Wkbj79 (1863) |
Numerical id | 18 |
Author | Wkbj79 (1863) |
Entry type | Definition |
Classification | msc 51A10 |
Classification | msc 15A04 |
Classification | msc 51A15 |
Related topic | DihedralGroup |
Related topic | DeterminingRotationsAndReflectionsInMathbbR2 |
Defines | symmetry about |
Defines | symmetric![]() |
Defines | symmetric about |
Defines | rotational symmetry |
Defines | point symmetry |
Defines | symmetry about a point |
Defines | symmetric about a point |
Defines | reflectional symmetry |
Defines | line symmetry |
Defines | symmetry about a line |
Defines | symmetric about a line |