{"id":7916,"date":"2014-02-04T20:28:57","date_gmt":"2014-02-04T19:28:57","guid":{"rendered":"http:\/\/vivesur.com\/wpblog\/?p=7916"},"modified":"2014-02-04T20:28:57","modified_gmt":"2014-02-04T19:28:57","slug":"clasificacion-simetrica-prehistorica","status":"publish","type":"post","link":"https:\/\/www.vivesur.com\/wpblog\/clasificacion-simetrica-prehistorica\/","title":{"rendered":"Clasificacion simetrica prehistorica"},"content":{"rendered":"<p><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-il2-2014-02-4-20-28.gif\" alt=\"wpid-il2-2014-02-4-20-28.gif\" width=\"205\" height=\"205\" \/><\/span><\/p>\n<p>\u00a0\u00a0\u00a0<strong>Chapter 1.3<br \/>\n<\/strong><br \/>\n\u00a0<span style=\"color: rgb(219,43,23);\"><strong>Classification of Symmetry <\/strong><\/span>?\u00a0<span style=\"color: rgb(219,43,23);\"><strong>Transformations and Groups<\/strong><\/span><\/p>\n<p>As the basis for the <em><strong>classification of the symmetry groups<\/strong><\/em> <em>G<\/em> three elements were taken into consideration: the types of symmetries (isometries, similarity symmetries, conformal symmetries) that occur in <em>G<\/em>, the space on which the group <em>G<\/em> acts, and the sequence of maximal included proper subspaces, invariant with respect to the group <em>G<\/em>. According to this, the <em><strong>Bohm symbols<\/strong><\/em> (<span style=\"text-decoration: underline;\"><a href=\"http:\/\/www.emis.de\/monographs\/jablan\/refer.htm#bo66\">J.\u00a0Bohm, K.\u00a0Dornberger-Schiff, 1966<\/a><\/span>) are used for the categorization of the groups of isometries. Symbols of the same type are applied to the similarity symmetry and conformal symmetry groups. For example, the symmetry group of square <strong>D<span style=\"vertical-align: sub;\">4<\/span><\/strong> acts in plane and possesses only one invariant point, so it belongs to the category <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span> &#8211; the symmetry groups of rosettes.<br \/>\nA transformation <em>S<\/em> is an <em><strong>isometry<\/strong><\/em> of certain space <em>En<\/em> (<em>Sn<\/em>) if for every two points <em>P<\/em>, <em>Q<\/em> of that space |<em>P<\/em>,Q| = |<em>S<\/em>(<em>P<\/em>),<em>S<\/em>(<em>Q<\/em>)| holds, where |<em>P<\/em>,<em>Q<\/em>| denotes the length of the line segment defined by points <em>P<\/em>, <em>Q<\/em>. All isometries of some space form a group.<br \/>\nA transformation <em>S<\/em> of <em>n<\/em>-dimensional space is called <em><strong>indirect<\/strong><\/em> (or reflective, sense reversing, opposite, odd) if it transforms any oriented (<em>n<\/em>+1)-point system onto an oppositely oriented (<em>n<\/em>+1)-point system (line segment <em>AB<\/em> onto <em>BA<\/em>, triangle <em>ABC<\/em> onto <em>ACB<\/em>, tetrahedron <em>ABCD<\/em> onto <em>ACBD<\/em> in cases of <em>n<\/em> = 1,2,3 respectively). Otherwise, it is called <em><strong>direct<\/strong><\/em> (sense preserving, even) (Figure 1.6).<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-16-2014-02-4-20-28.jpg\" alt=\"wpid-16-2014-02-4-20-28.jpg\" width=\"628\" height=\"188\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.6<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">(a) Direct and (b) indirect plane isometry.\n<\/td>\n<\/tr>\n<\/table>\n<p>As an elementary isometric transformation we can take the <em><strong>reflection<\/strong><\/em>, non-identical isometry of space <em>En<\/em> (<em>Sn<\/em>) for which, every point of its subspace <em>En<\/em><span style=\"vertical-align: super;\">-1<\/span> (<em>Sn<\/em><span style=\"vertical-align: super;\">-1<\/span>) is an invariant point. In particular, for <em>n<\/em> = 1 we have <em><strong>point reflection<\/strong><\/em>, for <em>n<\/em> = 2 <em><strong>line reflection<\/strong><\/em> (or simply &#8211; reflection), for <em>n<\/em> = 3 <em><strong>plane reflection<\/strong><\/em>, involutional indirect isometries. According to the fundamental theorem on <em><strong>minimal<\/strong><\/em> or <em><strong>canonic representation<\/strong><\/em> of an isometric transformation of space <em>En<\/em> (<em>Sn<\/em>), which states that every isometry of this space can be presented as a composition of maximum <em>n<\/em>+1 (plane) reflections, it is possible to classify the isometries of different spaces.<br \/>\nThe classification of isometric transformations and corresponding symmetry groups is common for spaces <em>En<\/em>, <em>Sn<\/em>, <em>Ln<\/em> for <em>n<\/em> &lt; 2, while for <em>n<\/em>  2 different possibilities of relations of disjoint lines, which are defined by the axiom of parallelism, condition specific differences. This work exclusively discusses Euclidean spaces.<br \/>\nIn the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> (plane) we distinguish the following isometric transformations (Figure 1.7):<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-17-2014-02-4-20-28.jpg\" alt=\"wpid-17-2014-02-4-20-28.jpg\" width=\"667\" height=\"558\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.7<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">(a) Identity transformation; (b) reflection; (c) rotation; (d) translation; (e) glide reflection.\n<\/td>\n<\/tr>\n<\/table>\n<p>1) <em><strong>identity transformation<\/strong><\/em> <em>E<\/em>, with the minimal reflectional representation of the length 2 (<em>R<\/em><span style=\"vertical-align: super;\">2<\/span> = <em>E<\/em>);<br \/>\n2) <em><strong>reflection<\/strong><\/em> <em>R<\/em>;<br \/>\n3) <em><strong>rotation<\/strong><\/em> <em>S<\/em> = <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span><em>R<\/em><span style=\"vertical-align: sub;\">2<\/span>, the product of two reflections in the reflection lines crossing in the invariant point (center of rotation). The oriented angle of rotation is equal to twice the angle between the reflection lines <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span>, <em>R<\/em><span style=\"vertical-align: sub;\">2<\/span>;<br \/>\n4) <em><strong>translation<\/strong><\/em> <em>X<\/em> = <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span><em>R<\/em><span style=\"vertical-align: sub;\">2<\/span>, the product of two reflections with parallel reflection lines, such that the translation vector is perpendicular to them and equal to twice the oriented distance between the reflection lines <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span>, <em>R<\/em><span style=\"vertical-align: sub;\">2<\/span>;<br \/>\n5) <em><strong>glide reflection<\/strong><\/em> <em>P<\/em> = <em>R<\/em><span style=\"vertical-align: sub;\">3<\/span><em>X<\/em> = <em>XR<\/em><span style=\"vertical-align: sub;\">3<\/span> = <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span><em>R<\/em><span style=\"vertical-align: sub;\">2<\/span><em>R<\/em><span style=\"vertical-align: sub;\">3<\/span>, the commutative product of a translation <em>X<\/em> and a reflection <em>R<\/em><span style=\"vertical-align: sub;\">3<\/span> with the reflection line parallel to the translation axis.<br \/>\nWith respect to the invariant figures, all the points of the plane <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> are invariant points of the identity transformation <em>E<\/em>, reflection <em>R<\/em> maintains the invariance of all the points of the reflection line, rotation <em>S<\/em> possesses a single invariant point &#8211; the center of rotation, while translation and glide reflection have no invariant points. A glide reflection possesses a single invariant line &#8211; the axis, and a translation keeps invariant all the lines parallel to the translation axis.<br \/>\nIn the case of rotation, if the relation <em>S<\/em> = <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span><em>R<\/em><span style=\"vertical-align: sub;\">2<\/span> = <em>R<\/em><span style=\"vertical-align: sub;\">2<\/span><em>R<\/em><span style=\"vertical-align: sub;\">1<\/span> holds, i.e. if the reflection lines <em>R<\/em><span style=\"vertical-align: sub;\">1<\/span>, <em>R<\/em><span style=\"vertical-align: sub;\">2<\/span> are perpendicular, as a result we get the special involutional rotation &#8211; <em><strong>central reflection<\/strong><\/em> <em>Z<\/em> (two-fold rotation, half-turn, point-reflection) (Figure 1.8).<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-18-2014-02-4-20-28.jpg\" alt=\"wpid-18-2014-02-4-20-28.jpg\" width=\"370\" height=\"204\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.8<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">Central reflection <em>Z<\/em>.\n<\/td>\n<\/tr>\n<\/table>\n<p>When orientation is considered we distinguish <em><strong>direct transformations<\/strong><\/em> (or sense preserving transformations): identity transformation <em>E<\/em>, translation, rotation, and <em><strong>indirect transformations<\/strong><\/em> (sense reversing transformations): reflection and glide reflection. Since direct transformations are the product of an even, and the indirect ones of an odd number of reflections, we can call them respectively <em><strong>even<\/strong><\/em> and <em><strong>odd transformations<\/strong><\/em>.<br \/>\nIf a symmetry transformation <em>S<\/em> can be represented as a composition <em>S<\/em> = <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>Sn<\/em> such that <em>SiSj<\/em> = <em>SjSi<\/em>, <em>i<\/em>,<em>j<\/em> = 1,,<em>n<\/em>, we can call it a <em><strong>complex<\/strong><\/em> or <em><strong>composite transformation<\/strong><\/em> while the transformations <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span>,,<em>Sn<\/em> we call the <em><strong>dependent transformations<\/strong><\/em> or dependent elements of symmetry. We will use such approach whenever we are interested to learn to what degree the dependent elements of symmetry influence the characteristics of the composite transformation, and whether they have lost or preserved their geometric and visual characteristics during it. For example, a glide reflection is such commutative composition of translation and reflection, with reflection line parallel to the translation axis.<br \/>\nAn analogous procedure makes possible the classification of isometries of the space <em>E<\/em><span style=\"vertical-align: super;\">3<\/span>, where each isometry can be represented as the composition of four plane reflections at the most. Besides the transformations of the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> afore mentioned with the line reflections substituted by plane reflections, as the new transformations of the space <em>E<\/em><span style=\"vertical-align: super;\">3<\/span> we have two more transformations. They are a direct isometry &#8211; <em><strong>twist<\/strong><\/em> (screw), the commutative composition of a rotation and a translation, the canonic representation of which consists of four plane reflections and indirect isometry &#8211; <em><strong>rotatory reflection<\/strong><\/em>, the commutative composition of a rotation and a plane reflection in the plane perpendicular to the rotation axis, the canonic representation of which consists of three plane reflections. In particular, the involutional rotatory reflection, which is the composition of three plane reflections of which every two commute, is called <em><strong>point inversion<\/strong><\/em> <em>Z<\/em> (or rotatory inversion).<br \/>\nFor every element <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span> of a transformation group <em>G<\/em> we can define the <em><strong>conjugate<\/strong><\/em> of the element <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span> by an element <em>S<\/em> as the product <em>S<\/em><span style=\"vertical-align: super;\">-1<\/span><em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em>, which we denote by <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em>. If <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em>  <em>G<\/em>, then the mapping <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span> onto <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em> represents an automorphism of the group <em>G<\/em>. If the element <em>S<\/em> by means of which this automorphism is being realized belongs to the group <em>G<\/em>, such an automorphism is called an <em><strong>internal automorphism<\/strong><\/em>. Any other automorphism of a group <em>G<\/em> is called an <em><strong>external automorphism<\/strong><\/em>. An important characteristic of a conjugate is that the order of the conjugate <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em> is equal to the order of the element <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span>. If a figure <em>f<\/em> is invariant under a transformation <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span>, then <em>S<\/em>(<em>f<\/em>) is the figure invariant under transformation <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em>. The conjugate of a reflection <em>R<\/em> with invariant reflection line <em>p<\/em>, derived by isometry <em>S<\/em>, is the reflection <em>RS<\/em> with the invariant reflection line <em>S<\/em>(<em>p<\/em>). Hence we can conclude that the isometry <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span> and all its conjugates <em>S<\/em><span style=\"vertical-align: sub;\">1<\/span><em>S<\/em> derived by different isometries <em>S<\/em> constitute one <em><strong>class of equivalence<\/strong><\/em>, i.e. the class of isometries having the same name, which means that (internal) automorphism of a group of isometries <em>G<\/em> transforms reflections onto reflections, rotations onto rotations, etc. The properties of the (internal) automorphisms are frequently used when proving theorems on isometric transformations and the other symmetry transformations. For example, by <em>gR<\/em>, <em>g<\/em>  <em>G<\/em>, is defined an internal automorphism of the symmetry group of square <em>G<\/em>, given by presentation<br \/>\n{<em>S<\/em>,<em>R<\/em>} \u00a0\u00a0\u00a0 <em>S<\/em><span style=\"vertical-align: super;\">4<\/span> = <em>R<\/em><span style=\"vertical-align: super;\">2<\/span> = (<em>RS<\/em>)<span style=\"vertical-align: super;\">2<\/span> = <em>E<\/em>: \u00a0\u00a0\u00a0<em>ER<\/em> = <em>E<\/em>,\u00a0\u00a0\u00a0 <em>RR<\/em> = <em>R<\/em>,\u00a0\u00a0\u00a0 (<em>RS<\/em>)<em>R<\/em> = <em>SR<\/em>, ?\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 (<em>SR<\/em>)<em>R<\/em> = <em>RS<\/em>, \u00a0\u00a0\u00a0<em>SR<\/em> = <em>S<\/em><span style=\"vertical-align: super;\">3<\/span>, \u00a0\u00a0\u00a0 (<em>S<\/em><span style=\"vertical-align: super;\">2<\/span>)<em>R<\/em> = <em>S<\/em><span style=\"vertical-align: super;\">2<\/span>, \u00a0\u00a0\u00a0 (<em>S<\/em><span style=\"vertical-align: super;\">3<\/span>)<em>R<\/em> = <em>S<\/em>.<br \/>\nIn the same way, it is defined an external automorphism of the rotational group of square <em>H<\/em>, given by presentation<br \/>\n{<em>S<\/em>} \u00a0\u00a0\u00a0 <em>S<\/em><span style=\"vertical-align: super;\">4<\/span> = <em>E<\/em>, \u00a0\u00a0\u00a0<em>SR<\/em> = <em>S<\/em><span style=\"vertical-align: super;\">3<\/span>, \u00a0\u00a0\u00a0 (<em>S<\/em><span style=\"vertical-align: super;\">2<\/span>)<em>R<\/em> = <em>S<\/em><span style=\"vertical-align: super;\">2<\/span>, \u00a0\u00a0\u00a0 (<em>S<\/em><span style=\"vertical-align: super;\">3<\/span>)<em>R<\/em> = <em>S<\/em>,<br \/>\nwhere the reflection line of reflection <em>R<\/em> contains the center of four-fold rotation <em>S<\/em>. Hence, external automorphisms are very efficient tool for extending symmetry groups.<br \/>\nSince the product of direct transformations is a direct transformation, and the inverse of a direct transformation is a direct transformation, each group of transformations <em>G<\/em>, which contains at least one indirect transformation has a subgroup of the index 2, denoted by <em>G<\/em><span style=\"vertical-align: super;\">+<\/span>, which consists of direct transformations of the group <em>G<\/em>. For example, the rotational subgroup of sqare <em>H<\/em> satisfies this condition regarding the symmetry group of square, so <em>H<\/em> = <em>G<\/em><span style=\"vertical-align: super;\">+<\/span>, [<em>G<\/em>:<em>H<\/em>] = 2. All direct isometries of the space <em>En<\/em> can be identified as movements of a material object in the space <em>En<\/em>, as opposed to indirect isometries which do not have such a physical interpretation (e.g., a plane reflection does not represent motion in <em>E<\/em><span style=\"vertical-align: super;\">3<\/span>).<br \/>\nFor a figure <em>f<\/em> with the symmetry group <em>Gf<\/em>, which consists only of direct symmetries, it is possible to have the <em><strong>enantiomorphism<\/strong><\/em> &#8211; <em><strong>enantiomorphic modifications<\/strong><\/em> of a figure <em>f<\/em>, i.e. to have the \u201cleft\u201d and \u201cright\u201d form of the figure <em>f<\/em> (Figure 1.9). The existence of indirect symmetries of a figure <em>f<\/em> implies the absence of enantiomorphism.<br \/>\nSince reflections have a role of elementary isometric transformations, while all other isometries are their finite compositions, of special interest will be symmetry <em><strong>groups generated by reflections<\/strong><\/em> &#8211; groups, a set of generators of which consists exclusively of reflections. Since every reflection keeps invariant each point of the reflection line, the fundamental region of these groups will possess a fixed shape, will not allow variations and will have rectilinear edges. All symmetry groups will be subgroups of groups generated by reflections. In the case of conformal symmetry groups, along with reflections, circle inversions have the analogous function. For example, the symmetry group of square is the group generated by reflections, with the fundamental region of the fixed shape (Figure 1.3a).<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-19-2014-02-4-20-28.jpg\" alt=\"wpid-19-2014-02-4-20-28.jpg\" width=\"378\" height=\"221\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.9<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">&#8220;Left&#8221; and &#8220;right&#8221; rosette with the symmetry group <strong>C<span style=\"vertical-align: sub;\">4<\/span><\/strong>, consisting of direct symmetries.\n<\/td>\n<\/tr>\n<\/table>\n<p>The results of composition of plane isometries are different <em><strong>categories of groups<\/strong><\/em> of isometries of the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>, represented by Bohm symbols as: <em>G<\/em><span style=\"vertical-align: sub;\">210<\/span> &#8211; symmetry groups of <em><strong>finite friezes<\/strong><\/em>, <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span> &#8211; symmetry groups of <em><strong>rosettes<\/strong><\/em>, <em>G<\/em><span style=\"vertical-align: sub;\">21<\/span> &#8211; symmetry groups of <em><strong>friezes<\/strong><\/em> and <em>G<\/em><span style=\"vertical-align: sub;\">2<\/span> &#8211; symmetry groups of <em><strong>ornaments<\/strong><\/em>. Because of the relation <em>G<\/em><span style=\"vertical-align: sub;\">210<\/span>  <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span>, in this work we will discuss only the categories <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span>, <em>G<\/em><span style=\"vertical-align: sub;\">21<\/span>, <em>G<\/em><span style=\"vertical-align: sub;\">2<\/span>, while the category <em>G<\/em><span style=\"vertical-align: sub;\">210<\/span> will be discussed within the category <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span>. The <em><strong>definitions of symmetry groups<\/strong><\/em> will be derived directly from Bohm symbols: symmetry groups of rosettes are groups of isometries of the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> (plane) with an invariant 0-dimensional subspace (point), symmetry groups of friezes are groups of isometries of the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> with an invariant 1-dimensional subspace (line) and without invariant points, while symmetry groups of ornaments are groups of isometries of the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> without invariant subspaces (points, lines). The groups of the category <em>Gn<\/em> are called the <em><strong>space groups<\/strong><\/em>, the groups of the category <em>Gn<\/em><span style=\"vertical-align: sub;\">1<\/span> the <em><strong>line groups<\/strong><\/em>, and the groups of the category <em>Gn<\/em><span style=\"vertical-align: sub;\">0<\/span> the <em><strong>point groups<\/strong><\/em> of the space <em>En<\/em>. With symmetry groups of friezes <em>G<\/em><span style=\"vertical-align: sub;\">21<\/span> and symmetry groups of ornaments <em>G<\/em><span style=\"vertical-align: sub;\">2<\/span>, a group contains one or two generating translations respectively, so that each of these groups has a <em><strong>translational subgroup<\/strong><\/em>. A <em><strong>lattice<\/strong><\/em> is the orbit of a point with respect to a discrete group of translations. For the friezes it is a linear series of equidistant points while for ornaments we get a <em><strong>plane lattice<\/strong><\/em> or simply a lattice. Five different symmetry types of plane lattices bear the name of <em><strong>Bravais lattices<\/strong><\/em>; the points of these lattices are defined by five different isohedral tessellations, which consist of parallelograms, rhombuses, rectangles, squares or regular hexagons. To Bravais lattices correspond the <em><strong>crystal systems<\/strong><\/em> of the same names (Figure 1.10).<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-110-2014-02-4-20-28.gif\" alt=\"wpid-110-2014-02-4-20-28.gif\" width=\"515\" height=\"293\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.10<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">Five plane Bravais lattices.\n<\/td>\n<\/tr>\n<\/table>\n<p>Because the symmetry groups of friezes <em>G<\/em><span style=\"vertical-align: sub;\">21<\/span> are groups of isometries of the plane <em>E<\/em><span style=\"vertical-align: super;\">2<\/span> with an invariant line, they cannot have rotations of an order greater than 2.<br \/>\nFor the symmetry groups of ornaments <em>G<\/em><span style=\"vertical-align: sub;\">2<\/span> so-called <em><strong>crystallographic restriction<\/strong><\/em> holds, according to which symmetry groups of ornaments can have only rotations of the order <strong>n<\/strong>=1,2,3,4,6. The term \u201c<em><strong>crystallographic groups<\/strong><\/em>\u201d is used for all groups which satisfy this condition, despite the category they belong to.<br \/>\nIn isometry groups all distances between points under the effect of symmetries remain unchanged and the <em><strong>congruence<\/strong><\/em> of homologous figures is preserved. Consequently, the same holds for all other geometric properties of such figures, so that the <em><strong>equiangularity<\/strong><\/em> (the congruence of the angles of homologous figures) and their <em><strong>equiformity<\/strong><\/em> (the same form of homologous figures) are the direct consequences of isometrism.<br \/>\nThe next class of symmetry groups we shall consider are the <em><strong>similarity symmetry groups<\/strong><\/em>. A <em><strong>similarity transformation<\/strong><\/em> of the space <em>En<\/em> is a transformation which to each line segment of length <em>AB<\/em> assigns a line segment of the length <em>kAB<\/em> whereby <em>k<\/em> is a real positive number, the <em><strong>coefficient of similarity<\/strong><\/em>. In particular, if <em>k<\/em> = 1 then a similarity transformation is an isometry. According to the theorem on the existence of an invariant point of every similarity transformation which is not an isometry, there are, besides isometries, three types of similarity symmetry transformations of the space <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>:<br \/>\n(i) <em><strong>central dilatation<\/strong><\/em> <em>K<\/em> (or simply dilatation), a transformation which to each vector (<em>A<\/em>,<em>B<\/em>) assigns the vector (<em>A<\/em>\u2019,<em>B<\/em>\u2019), such that <em>A<\/em>\u2019 = <em>K<\/em>(<em>A<\/em>), <em>B<\/em>\u2019 = <em>K<\/em>(<em>B<\/em>) and (<em>A<\/em>\u2019,<em>B<\/em>\u2019) = <em>k<\/em>(<em>A<\/em>,<em>B<\/em>), where the coefficient of the dilatation is <em>k<\/em>  \\{-1,0,1} ;<br \/>\n(ii) <em><strong>dilative rotation<\/strong><\/em> <em>L<\/em>, the commutative composition of a central dilatation <em>K<\/em> and a rotation, with a common invariant point;<br \/>\n(iii) <em><strong>dilative reflection<\/strong><\/em> <em>M<\/em>, the commutative composition of a dilatation <em>K<\/em> and a reflection in the reflection line containing the invariant point (center) of the dilatation <em>K<\/em> (Figure 1.11).<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-111-2014-02-4-20-28.gif\" alt=\"wpid-111-2014-02-4-20-28.gif\" width=\"613\" height=\"242\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.11<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">(a) Dilatation; (b) dilative rotation; (c) dilative reflection.\n<\/td>\n<\/tr>\n<\/table>\n<p>Those transformations are, in the given order, isomorphic with the isometries of the space <em>E<\/em><span style=\"vertical-align: super;\">3<\/span>: translation, twist and glide reflection. They make possible the extension of the symmetry groups of rosettes <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span> by the external automorphism, having as the result <em><strong>similarity symmetry groups<\/strong><\/em> <em>S<\/em><span style=\"vertical-align: sub;\">20<\/span> that we will, thanks to the existence of the invariant point, call the <em><strong>similarity symmetry groups of rosettes<\/strong><\/em>.<br \/>\nDilatations <em>K<\/em> and dilative rotations <em>L<\/em> are direct, while dilative reflections <em>M<\/em> are indirect transformations. They all possess the properties of equiangularity and equiformity. All other aspects of similarity symmetry groups (the problems of enantiomorphism, fundamental regions, tessellations,) will be discussed analogously to the case of isometry groups.<br \/>\nFurther generalization leads to <em><strong>conformal transformations<\/strong><\/em> or circle preserving transformations of the plane <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>\\{<em>O<\/em>} ; for them the property of equiangularity has been preserved, but not that of equiformity. We have, as the elementary transformation of conformal symmetry in <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>\\{<em>O<\/em>} , the <em><strong>circle inversion<\/strong><\/em> <em>R<\/em><span style=\"vertical-align: sub;\">I<\/span> (or simply inversion) &#8211; an involutional transformation isomorphic with a reflection, that gives to each point <em>A<\/em> in the plane <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>\\{<em>O<\/em>} a point <em>A<\/em><span style=\"vertical-align: sub;\">1<\/span> so that (<em>O<\/em>,<em>A<\/em>)(<em>O<\/em>,<em>A<\/em><span style=\"vertical-align: sub;\">1<\/span>) = <em>r<\/em><span style=\"vertical-align: super;\">2<\/span>, where <em>r<\/em> is the radius of the <em><strong>inversion circle<\/strong><\/em> <em>c<\/em>(<em>O<\/em>,<em>r<\/em>) and <em>O<\/em> is the singular point of the plane <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>\\{<em>O<\/em>} (Figure 1.12). Just like a reflection, for which each point of the reflection line is invariant, an inversion maintains invariant each point of the inversion circle. By discussing a line as a circle with an infinite radius (and treating as circles, at the same time and under the same term, circles and lines) it is possible to identify reflections with circle inversions. In such a case, all circle inversions (including line reflections) and their compositions, can be discussed as <em><strong>circle preserving transformations<\/strong><\/em>, i.e. transformations mapping circles (including lines) onto circles.<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-112-2014-02-4-20-28.jpg\" alt=\"wpid-112-2014-02-4-20-28.jpg\" width=\"278\" height=\"269\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.12<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">Circle inversion.\n<\/td>\n<\/tr>\n<\/table>\n<p>Besides the circle inversion <em>R<\/em><span style=\"vertical-align: sub;\">I<\/span>, by composing it with isometries maintaining invariant the circle line <em>c<\/em> of the inversion circle <em>c<\/em>(<em>O<\/em>,<em>r<\/em>) &#8211; with a reflection with reflection line containing the circle center <em>O<\/em> or with a rotation with the rotation center <em>O<\/em>, we have two more conformal transformations:<br \/>\n(i) <em><strong>inversional reflection<\/strong><\/em> <em>Z<\/em><span style=\"vertical-align: sub;\">I<\/span> = <em>R<\/em><span style=\"vertical-align: sub;\">I<\/span><em>R<\/em> = <em>RR<\/em><span style=\"vertical-align: sub;\">I<\/span>, the involutional transformation, the commutative composition of a reflection and a circle inversion;<br \/>\n(ii) <em><strong>inversional rotation<\/strong><\/em> <em>S<\/em><span style=\"vertical-align: sub;\">I<\/span> = <em>SR<\/em><span style=\"vertical-align: sub;\">I<\/span> = <em>R<\/em><span style=\"vertical-align: sub;\">I<\/span><em>S<\/em>, the commutative composition of a rotation and a circle inversion (Figure 1.13).<br \/>\n?<\/p>\n<p style=\"text-align: center\"><span style=\"font-size: 12pt;\"><img loading=\"lazy\" src=\"http:\/\/vivesur.com\/wpblog\/wp-content\/uploads\/2014\/02\/wpid-113-2014-02-4-20-28.jpg\" alt=\"wpid-113-2014-02-4-20-28.jpg\" width=\"545\" height=\"313\" \/><\/span><\/p>\n<p style=\"text-align: center\">Figure 1.13<\/p>\n<table>\n<tr>\n<td style=\"width: 355px;padding: 1px,1px,1px,1px;border-top: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);border-bottom: 0px solid rgb(138975,79815,0);border-right: 0px solid rgb(138975,79815,0);margin: 0px,0px,0px,0px;\" style=\"text-align: center\">(a) Inversional reflection; (b) inversional rotation.\n<\/td>\n<\/tr>\n<\/table>\n<p>Those three conformal symmetry transformations, besides isometries and similarity symmetry transformations, constitute the finite and infinite <em><strong>conformal symmetry groups<\/strong><\/em> <em>C<\/em><span style=\"vertical-align: sub;\">21<\/span>, <em>C<\/em><span style=\"vertical-align: sub;\">2<\/span> &#8211; <em><strong>conformal symmetry groups of rosettes<\/strong><\/em> in <em>E<\/em><span style=\"vertical-align: super;\">2<\/span>\\{<em>O<\/em>} .<br \/>\nAs an extension of the symmetry groups of rosettes <em>G<\/em><span style=\"vertical-align: sub;\">20<\/span> we have the <em><strong>finite conformal symmetry groups<\/strong><\/em> <em>C<\/em><span style=\"vertical-align: sub;\">21<\/span> isomorphic with the symmetry groups of <em><strong>tablets<\/strong><\/em> <em>G<\/em><span style=\"vertical-align: sub;\">320<\/span>. As a further extension of finite conformal symmetry groups <em>C<\/em><span style=\"vertical-align: sub;\">21<\/span> by the similarity symmetry transformations <em>K<\/em>, <em>L<\/em>, <em>M<\/em>, we get the <em><strong>infinite conformal symmetry groups<\/strong><\/em> <em>C<\/em><span style=\"vertical-align: sub;\">2<\/span>. The similarity symmetry groups <em>S<\/em><span style=\"vertical-align: sub;\">20<\/span> and the infinite conformal symmetry groups <em>C<\/em><span style=\"vertical-align: sub;\">2<\/span> are isomorphic with the line symmetry groups of the space <em>E<\/em><span style=\"vertical-align: super;\">3<\/span> &#8211; the symmetry groups of <em><strong>rods<\/strong><\/em> <em>G<\/em><span style=\"vertical-align: sub;\">31<\/span>. In line with the isomorphism mentioned, all similarity symmetry and conformal symmetry transformations offer a reflectional (canonic) representation by, at most, four reflections (reflections and circle inversions). By applying this isomorphism, ornamental motifs which correspond to the similarity symmetry and conformal symmetry groups, satisfy one more scope of painting: adequate interpretation of space objects in the plane. The plane structures obtained are called <em><strong>generalized projections<\/strong><\/em> of the symmetry groups of tablets <em>G<\/em><span style=\"vertical-align: sub;\">320<\/span> and rods <em>G<\/em><span style=\"vertical-align: sub;\">31<\/span>.<\/p>\n<p>de <a href=\"http:\/\/www.emis.de\/monographs\/jablan\/chap13.htm#i180\">http:\/\/www.emis.de\/monographs\/jablan\/chap13.htm#i180<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u00a0\u00a0\u00a0Chapter 1.3 \u00a0Classification of Symmetry ?\u00a0Transformations and Groups As the basis for the classification of the symmetry groups G three elements were taken into consideration: the types of symmetries (isometries,&hellip; <a href=\"https:\/\/www.vivesur.com\/wpblog\/clasificacion-simetrica-prehistorica\/\" class=\"apace-readmore-link\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[1,8],"tags":[],"_links":{"self":[{"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/posts\/7916"}],"collection":[{"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/comments?post=7916"}],"version-history":[{"count":0,"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/posts\/7916\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/media?parent=7916"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/categories?post=7916"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.vivesur.com\/wpblog\/wp-json\/wp\/v2\/tags?post=7916"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}