As an illustration of the methodological approach used in this work, we will give the example of one symmetry group and its complete comparative analysis from the point of view of the theory of symmetry and ornamental art.
Let the discrete group of isometric transformations of the plane E2, generated by a glide reflection P and a reflection R1, be given by the presentation:
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The group discussed possesses an invariant space E2 - the plane, an invariant subspace E1 - a line, and has no invariant points. Therefore it belongs to the category of symmetry groups of friezes G21 - the line groups of the plane E2 (S2) without invariant points. Distinguishing between the spaces E2, S2, L2 is not necessary because we are dealing with the line groups. Because this group is generated by a glide reflection P perpendicular to the reflection R1, its crystallographic symbol will be pmg, or in short form mg (M. Senechal, 1975). Within the crystallographic symbol pmg, p denotes the presence of a translation X = P2, i.e. the translational subgroup 11={X}; the symbol m denotes a reflection R1 perpendicular to this translation, and the symbol g denotes the glide reflection P. In the short symbol mg, the translation symbol p is omitted.
Since the set P,R1 is a generator set of the group mg, after concluding that the reflection line of R1 is perpendicular to the axis of the glide reflection P, we can construct an appropriate ornamental motif, the visual model of the frieze symmetry group mg. This is achieved by applying the transformations P and R1 to the chosen asymmetric figure, which belongs to a fundamental region of the symmetry group mg (Figure 1.19).
Representing the glide reflection P as the commutative
composition X1R3 = R3X1 of a translation X1 = R1R2
(composition of reflections R1, R2 with parallel reflection
lines) and a reflection R3 with the reflection line parallel
to the axis of the translation X1, we come to the conclusion
that the product R1P = R1R1R2R3 =
R2R3 is the commutative
composition of perpendicular reflections R2, R3, i.e. a
half-turn T. The conjugates of
reflection R1 and half-turn
T, derived by the powers of the glide reflection P,
define respectively the set of reflections with equidistant
reflection lines parallel to the reflection line of R1, and the set of
rotations of the order 2, where the distance between the
neighboring reflection lines (rotation centers) is
|X1|. So that, we come to the diagrammatic
interpretation of the group mg - a table of graphic
symbols of symmetry elements, where the axis of the glide
reflection is indicated by the dotted line and by the vector of
translation, reflection lines by solid lines, and centers of
rotations of the order 2 by the symbol § (Figure 1.20).
Using the substitution T = R1P we come to an algebraic
equivalent of the previous presentation of the group mg -
a new presentation of the same group:
Instead of the asymmetric figure, which under the action
of the group mg gives the frieze pattern, by considering
the orbit of the closure of a fundamental region of the group
mg we obtain the corresponding frieze tiling. The
fundamental region of the group mg and all other frieze
symmetry groups, is unbounded and allows the variation of all
boundaries which do not belong to reflection lines. Figure 1.21
shows two of these possibilities.
The Cayley diagram of the group mg is derived as
the orbit of a point in general position with respect to the
group mg. Instead of a direct mutual linking of all
vertexes (i.e. orbit points) and obtaining the complete graph, we
can, aiming for
simplification, link only the homologous points of the group
generators. By denoting with the broken oriented line the glide
reflection P, and with the dotted non-oriented line the
reflection R1, we get the Cayley diagram which corresponds to
the first presentation of the group mg (Figure 1.22a).
By an analogous procedure we come to the graph which
corresponds to its second presentation with the generator set
{R1,T}, where a half-turn is indicated with the dot-dash line
(Figure 1.22b).
Let us note also, that the defining relations can be read
off directly from the graph of the group. Each cycle, i.e. closed
path in which the beginning point coincides with the endpoint,
corresponds to a relation between the elements of the group and
vice versa. Cayley diagrams (graphs of the groups) may also very
efficiently serve to determine the subgroups of the given
symmetry group. Namely, every connected subgraph of the given
graph satisfying the following condition determines a certain
subgroup of the group discussed, and vice versa. The condition
in question is: an element (transformation) is included in the
subgraph either wherever it occurs, or not at all (i.e. it is
deleted). Of course, to be able to determine all the subgroups of
a given group, it is necessary to use its complete graph as the
basis for defining the subgraphs.
Since in the group mg there are indirect
isometries, this group does not give enantiomorphic
modifications. For the groups consisting only of direct
symmetries, the enantiomorphic modifications can be obtained by
applying the "left" (e.g., b) and "right" (d) form of an
elementary asymmetric figure. For example, for the group
11, generated by a translation X, this results in the
enantiomorphic friezes: bbbbbbbbbbbbbbbbbbbbbbbbbbbbb and
dddddddddddddddddddddddddddddd. The translation axis l of the
group mg is non-polar, because there exists an indirect
transformation, the reflection R1 for which the relation
R1(l) = -l holds.
Rotations of the order 2 in the group mg are polar because
each circle c drawn around the center of rotation of the order
2 is invariant only with respect to this rotation and to the
identity transformation E, so that the group C2 (
2) (generated by the half-turn T) of transformations
preserving the circle c invariant, a rosette subgroup
C2 (2) of the group mg, consists of direct
transformations. Besides the rosette subgroups C2 (
2), the group mg has also the rosette subgroups
D1 (m), namely the one generated by the reflection
R1, or by its conjugates.
The group mg contains as subgroups the following
symmetry groups of friezes: p1 (11) generated by the
translation X = P2, p1g (1g) generated by the glide
reflection P, pm1 (m1) generated by the translation
X and the reflection R1, and itself. Besides the list of all
frieze groups, subgroups of the group mg, the table of the
minimal indexes of subgroups of the given group points out the
possible desymmetrizations which lead to this subgroup. In
particular, considering the use of antisymmetry and color-
symmetry desymmetrizations, from this table we can see that
antisymmetry desymmetrizations of group mg result in the
subgroups of the index 2: 1g, 12 and m1. This
can be achieved by a black-white coloring (or, e.g., 1-2
indexing) according to the laws of antisymmetry, using the
following systems of (anti)generators: {P,e1R1} or
{e1R1,e1T} for obtaining the antisymmetry desymmetrization
mg/1g; {e1P,R1} or {R1,e1T} for
obtaining the antisymmetry desymmetrization mg/m1;
{e1P,e1R1} or {e1R1,T} for obtaining the
antisymmetry desymmetrization mg/12, where
e1 = (12), i.e. the group of color permutations PN = P2 = C2
(Figure 1.23).
The junior antisymmetry groups obtained can be understood
also as adequate visual interpretations of the symmetry groups of
bands G321 - as the Weber diagrams of the symmetry groups
of bands p2111, pm11 and p112 respectively.
In this case the alternation of colors white-black is understood
in the sense "above-under" the invariant plane of the frieze,
i.e. as the identification of the antiidentity transformation
e1 with the plane reflection in the invariant plane of the
group mg. The seven generating symmetry groups of friezes
G21, seven senior antisymmetry groups and seventeen junior
antisymmetry groups correspond to the 31 groups of symmetry of
bands, offering complete information on their presentations and
structures.
Using N = 4 colors and the system of colored generators
{c1P,c2R1} or
{c2R1,c1c2T}, we get the
color-symmetry desymmetrization mg/11, where
c1 = (12)(34) and c2 = (13)(24); hence, the group of color
permutations is PN = P4 = C2×C2 = D2 (Figure 1.24).
In all the antisymmetry and color-symmetry
desymmetrizations mentioned, for which the group PN is
regular, the subgroup H derived by the desymmetrization is a
normal subgroup of the group mg (1g, m1,
12, 12). Because of this, complete information on the
antisymmetry or colored symmetry group, i.e. on the corresponding
desymmetrization, is given by the number N and by the
group/subgroup symbol G/H. The next case of coloring with N = 3
colors, the irregular group PN and the subgroup H which is
not a normal subgroup of the group G, demands the symbols
G/H/H1. In this case, besides the number N, the group of
colored symmetry G*, i.e. the corresponding color-symmetry
desymmetrization is uniquely defined by the generating group G,
the stationary subgroup H of G*, which maintains every
individual index (color) unchanged and its symmetry subgroup
H1 which is the final result of the color-symmetry
desymmetrization. The index of the subgroup H in the group G
is equal to N and the product of the index of the subgroup
H1 in group H and the number N is equal to the order of
the group of color permutations PN, i.e. [G:H] = N,
[H:H1] = N1, and the order of the group PN is NN1.
As an example of the irregular case we can use the
color-symmetry desymmetrization of the group mg obtained by
N = 3 colors, i.e. by the system of colored generators:
{c1P,c2R1} or
{c2R1,c1c2T},
which results in
the color-symmetry desymmetrization mg/mg/1g,
where c1 = (123), c2 = (23), PN = P3 = D3 and [mg:
mg]=3, [mg:1g]=2. This color-symmetry
desymmetrization mg/mg/1g, N = 3 is shown on
Figure 1.25a, while the stationary subgroup H (mg) which
maintains each individual index (color) unchanged is singled out
on Figure 1.25b. All cases of subgroups which are not normal
subgroups of the given group are denoted in the tables of
(minimal) indexes of subgroups in groups by italic indexes (e.g.,
[mg:mg]=3).
In terms of construction, for frieze group mg we
can also distinguish the rosettal method of construction - the
multiplication of a rosette with the symmetry group C2
(2) (generated by the half-turn T) or D1 (
m) (generated by the reflection R1) by the glide reflection
P (Figure 1.26a, b). Like all other symmetry groups of friezes,
the group mg is the subgroup of the maximal symmetry
group of friezes mm generated by reflections. Since it is
the normal subgroup of the index 2, the antisymmetry
desymmetrization of the generating group mm with a set of
generators {X,R,R1} or {R,R1,R2} where X is the
translation, R the reflection in translation axis line, and
R1, R2 reflections with reflection lines perpendicular to
the translation axis, can be used.
By means of the system of (anti)generators:
{e1X,e1R,R1} =
{e1X, e1R,e1R1} or
{e1R,R1,e1R2} the antisymmetry desymmetrization
mm/mg
is obtained (Figure 1.27), where e1 = (12),
PN = P2 = C2, [mm:mg] = 2.
Many visual properties of the group mg, e.g., a
relative constructional and visual simplicity of corresponding
friezes conditioned by a high degree of symmetry, specific
balance of the stationariness conditioned by the presence of
reflections, by the non-polarity of the glide reflection axis, by
the absence of enantiomorphism, and the dynamism conditioned by
the presence of glide reflection and by polar, oriented
rotations, are the direct consequences of the algebraic-geometric
characteristics mentioned. Also, the different possibilities that
the group mg offers, e.g., the possibilities for
antisymmetry and color-symmetry desymmetrizations, the ways of
varying the form of the fundamental region, construction
possibilities etc., become evident after the analysis of this
symmetry group of friezes from the point of view of the theory of
symmetry.
Even such a concise illustration of the connections
between the theory of symmetry and ornamental art raises the
question of the place that ornamental art has today, from the
point of view of both the artist and the scientist. When
analyzing works of art, the approach to ornamental art from the
standpoint of the theory of symmetry offers the possibility for
serious analysis, a more profound study of the complete
historical development of ornamental art, the regularity and laws
on which the constructions of ornaments are founded, and an
efficient classification method of a large domain (isometric,
non-isometric, antisymmetry, color-symmetry ornamental motifs).
It opens for the artist a new field of exploration - a more
exact planning of visual effects, based on the knowledge of the
theory of symmetry and the psychology of visual perception. An
example of successful creativity, artistic imagination and
knowledge of exact geometric rules, is given by the work of
M.C.Escher, which points to the future of ornamental art as a
specific synthesis of science and art. On the other hand, to the
scientists of different disciplines, the theory of symmetry
offers various possibilities - to archaeologists an efficient
and reliable method of classification and comparative analysis;
to theorists of art the basis for working out exact aesthetic
criteria; to crystallographers, physicians and chemists an
obvious model of symmetry structures. Last, but not least, to
mathematicians ornamental art, as the treasury of the implicit
mathematical knowledge of humankind, represents an inspiring
field, rich with questions seeking an answer.
{R1,T} R12 = T2 = E.
{S1,S2} S12 = S22 =
(S1S2)n = E.
{S1,S2} S12 = S22 = E
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