 / 
Introduction
Weaving carpets of triangles into spheres and simpler polyhedra
Eliciting memorable spherically symmetrical semiregular polyhedra
Illustrative application to symbolic reconciliation
Spherical tiling and spherical polyhedra
Use of force directed layout to elicit memorable polyhedra
Potential significance of memorable irregularity?
References
Much is made of the recognition that everything is now connected to everything, especially as reflected in patterns of links in cyberspace (AlbertLaszlo Barabasi, Linked: how everything is connected to everything else and what it means for business, science, and everyday life, 2014; David Easley and Jon Kleinberg, Networks, Crowds, and Markets: reasoning about a highly connected world, 2010).
This insight has notably been explored through smallworld network graph theory and through understandings of "six degrees of separation", popularized by the parlour game Six Degrees of Kevin Bacon (Duncan J. Watts, Six Degrees: the science of a connected age, 2004). Various efforts at offering an idea of the complexity of this connectivity have been made based on internet traffic, web links, and the like.
Systematic efforts to document the connectivity of problems, strategies and international organizations as networks have been made in developing the online databases associated with the Encyclopedia of World Problems and Human Potential and the Yearbook of International Organizations. Extensive efforts have been made to analyze these networks and to find fruitful ways of visualizing them to render them more meaningful as a basis for new modes of action appropriate to the challenges of the times (Visualizing Relationship Networks: international, interdisciplinary, intersectorial, 1992; Visualizing Latent Significance in Patterns of Relationships: a case study in relative incompetence, 2012).
Considerable progress in this respect has been recently made in this respect by Tomá Fülöpp and Jacques de Mévius (Loop mining in the Encyclopedia of world problems, 2015), as reported at the conference on Futures Studies Tackling Wicked Problems (Turku, 2015). In the light of these efforts, the concern here is with the issue of how to render memorable and communicable the connectivity which emerges from such analysis. The concern is partially highlighted by possible responses to information overload (Optimizing Web Surfing Pathways for the Overloaded: polyhedral insights from the travelling salesman problem of operations research, 2015).
Web users have an increasing concern with how best to manage their web surfing experience given the constraints of time and information overload. The question explored here is a possible means of moving beyond a browser checklist of links ("favourites") and bookmarks, whether or not these are carefully nested within menus and organized by theme. This followed from earlier concern with the challenge of organizing relationships between websites in terms of the conversation threading of internet exchanges (Interweaving Thematic Threads and Learning Pathways: noonautics, magic carpets and wizdomes, 2010).
The complexity of some efforts at such network visualization has been deprecated through resulting in socalled "hairballs" (Graphs Beyond the Hairball; Lynn Cherny, Visualizing Graphs: beyond the "hairball", 2012; Martin Krzywinski, Hive Plots: Rational Network Visualization  Farewell to hairballs; Arlind Nocaj, et al, Untangling Hairballs, 2014; Stephen Few, From Giant Hairballs to Clear Patterns in Networks, Visual Business Intelligence, 2013; Hive plots and hairballs, Seeing Complexity: visualizing complex data, 2011; HansJörg Schulz and Christophe Hurter, Grooming the Hairball: how to tidy up network visualizations? 2013; Jeff Johnston, Embracing the Hairball, Exapative, 2015). Special software applications, such as sigma,js, have been developed to address the current challenge of graph drawing (Sigma.js Cleans up Hairball Network Visualizations). Hairballs are inherently unmemorable.
The particular focus here is on using analytical data on triangles of relationships in such a way as to construct memorable networks of these triangles based on any shared edges. This effectively weaves together those triangles detected into a "flat" 2dimensional "carpet". The further possibility then derives from detecting "folding" of that carpet into the form of a 3dimensional "bowl", as a consequence of connectivity at its outer edges forcing a degree of closure. As a result there is a "curling up" of the edges of the carpet with the possibility of detecting some such patterns which close completely to form a sphere. Approximations to such spherical closure, based on triangles, could take the form of polyhedra, whether simple or more complex  based on loops of more than three relationships (squares, pentagons, hexagons, etc).
Through their characteristic symmetry, this approach offers a means of detecting and communicating visually memorable forms of order within very complex networks  however they need to be labelled or colour coded. A particular feature of the approach is the use of forcedirected graph drawing, charactetistic of DataDriven Documents (d3.js), to elicit selforganizing convex polyhedra  without the conventional prerequisite for vertex coordinates.
Use is made of "hyperspace" in the title, rather than cyberspace, in order to stress the connectivity associated with hyperlinks within cyberspace. This follows from an early argument (From Information Highways to Songlines of the Noosphere: global configuration of hypertext pathways as a prerequisite for meaningful collective transformation, 1996). As such it offers an emphasis on conceptual entities and integrative cognitive engagement with them  hence its appriateness for consideration of problems, stategies, and the like, as noted above (Spherical Configuration of Interlocking Roundtables: Internet enhancement of global selforganization through patterns of dialogue, 1998).
Loop detection: The approach here is based on the analysis by Tomá Fülöpp using the strongly connected components algorithm of graph theory developed by Robert Tarjan (Depthfirst search and linear graph algorithms, SIAM Journal on Computing, 1972). This exercise was applied to networks of relationships between world problems perceived by international constituencies. It notably builds sets of loops linking three, four, five, and more, such entities  extending to hundreds. Such entities, potentially labelled (as shown below) are associated with the nodes.
Screen shots of vicious problem loops (or cycles) (screen shots from dynamic representations by Tomá Fülöpp, EWP Editing Platform) 

Selective view of 179 loops containing 3 nodes.  Selective view of 1212 loops containing 6 nodes. 
Clearly threefold loops correspond to a triangular pattern, fourfold to a square pattern, and fivefold to a pentagonal pattern. These can of course be recognized as the face components of the simpler polyhedra. Earlier work extended to exploration of links between problems and strategies (Feedback Loops Interlinking World Problems, 2000).
Loop interlocking: Earlier experiments focused on simple interlocking between polygons, as illustrated below (reproduced from World Problem Loop Interlocks, 2000)..
Screen shots of earlier experiments (representation of interlocking problem loops in virtual reality) 

In the recent analysis by Tomá Fülöpp (EWP Editing Platform, 2015), more systemic detection of loop interlocking has been enabled.
Polyhedral patterns: The question is how to employ these sets in a further step to build patterns of a higher order. In particular, is it possible to detect approximations to a sphere composed of a selected subset of the triangular patterns  and how is that subset to be detected? This implies ordering the triangles, as in a jigsaw puzzle, joining two of them on the basis of a shared edge (side)  meaning with distinct nodes at each end of the edge, common to the two triangles. Detection of subsets having such commonality is relatively trivial.
The challenge may be how to combine 2 triangles found in this way, with any larger pattern  excluding those which do not "fit" at any one stage in the process. The procedure may involve building up in "pools" of memory sets of 2 triangles, 3 triangles, and more, as pieces which may be useful at some stage in the elaboration of the larger pattern. Clearly many such patterns could be "under construction" in parallel in the course of the process  since different spheres might eventually emerge, at different stages after many iterations. Tomá Fülöpp already offers access to "interlocking loops".
As noted above, the process could detect simpler triangulated polyhedra where closure of the pattern is detected earlier in the selection  prior to those triangulated polyhedra which could take more spherical form of geodesic spheres  even more commonly recognized as geodesic domes (as noted below).
The process of fitting the triangles together is usefully illustrated by the following, prepared with Stella Polyhedron Navigator. Pairs of triangles sharing an edge can be identified (like Triangles A and B, A and C, and A and D). It is then a question detecting patterns of 4 triangles with one triangle in common (as in the case of Triangle A) in the image on the left below. Amongst that set, it is then a question of identifying those in which the "external" node is identical (as would be the case with Node 1 in the image on the left). Those meeting that condition then effectively form a tetrahedron of 4 triangles  and can be considered as having been folded into 3 dimensions as variously illustrated below.
Screen shots of stages in folding of simplest polyhedron variously sharing edges and nodes  
4 triangles in 2 dimensions  Stage in folding  Stage in folding 
Folding completed  Animation of folding process 
The possible relation to a sphere can be understood using a more complex polyhedron in the group of 5 Platonic polyhedra of which the tetrahedron above is the simplest. Again the argument is usefully illustrated by the following images (prepared with Stella Polyhedron Navigator). The degree of approximation to a sphere is evident in the animation on the right by addition of a circumsphere..
Illustration of process using animations of icosahedron 30 edges; 12 vertex nodes; 20 triangular faces (transparent) 

Carpet of triangles folding into approximation of a sphere as outmost edges merge together  Rotation of folded icosahedron (within a circumsphere) 
Geodesic spheres: Using the icosahedron as a base, further insights into the process of gathering triangles into a carpet pattern to be folded can be obtained by considering geodesic spheres. The animation (on the right below) shows the nature of the great circles formed by the edges coloured bluegrey, in contrast to the interrupted great circles by the edges coloured green.
Illustration of process using animations of 2frequency geodesic sphere based on icosahedron 120 edges (of 2 types); 42 vertex nodes (of 2 types); 80 triangular faces (of 2 types) 

Carpet of triangles folding into approximation of a sphere as outmost edges merge together  Rotation of geodesic sphere (2 edge types distinguished) 
The illustration may be repeated with the icosahedron as a base. The degree of approximation to a sphere increases by increasing the frequency and therefore the number of interrelated triangles required to constitute that pattern  as is evident in the image on the right below..
Illustration of process using animations of 3frequency geodesic sphere based on icosahedron 270 edges (of 5 types); 92 vertex nodes (of 3 types); 180 triangular faces (of 3 distinctly coloured types) 

Carpet of triangles folding into approximation of sphere as outmost edges merge together  Rotation of folded geodesic sphere (within a circumsphere) 
The focus above has been on triangles and the possibility of eliciting spherically symmetrical polyhedra based on triangulation (Triangulation of Incommensurable Concepts for Global Configuration, 2011. Also of interest is the possibility of detecting polyhedra that are somewhat irregular because their faces are not necessarily triangles. They may be squares, pentagons, hexagons, or the like. These notably characterize the 13 Archimedean polyhedra  somewhat less memorable (or wellknown) than the Platonic polyhedra. The relevance of spherically symmetrical polyhedral configuration to memorability has been argued separately (Memorable pattern language for configuring sets of websites? 2015)
Distinguishing methods for detecting polyhedra: In the light of the above description, various methods may be distinguished:
More complex procedures: The various procedures explored above take no account whatsoever of existing sophisticated mathematical algorithms by which patterns and cycles can be detected "through" numerous links, as discussed in the extensive literature in graph theory on Hamiltonian cycles. (Brent M. Dingle, Finding Hamiltonian Cycles in the Inner nCube, 1999; Carlo H. Séquin, Patterns on the Genus: 3 Klein Quartic; A. Ya. BelovKanel, et al Interlocking of Convex Polyhedra : towards a geometric theory of fragmented solids, Moscow Mathematical Journal, January 2009). Of related interest are insights from unfolding polyhedra (Brendan Lucier, Unfolding and Reconstructing Polyhedra, University of Waterloo, 2006). Through such procedures the detection of polyhedra could no doubt be rendered much more efficient.
Use of insights from spherical tiling are considered below.
Aspects of the methods above, and of possible applications, are usefully illustrated by a previous exercise (Middle East Peace Potential through Dynamics in Spherical Geometry: engendering connectivity from incommensurable 5fold and 6fold conceptual frameworks, 2012). Although discussed separately in detail, the approach can be briefly summarized by the following images. The argument there is focused on the symbolic significance associated with the 5fold and 6fold stars of Islam and Judaism in 2D  and how these might be reconciled in 3D.
Symbolic stars embedded in corresponding polygons  
5fold Star  6fold Star 
Truncated icosahedron "reconciling" pentagons and hexagons in 3D (images produced using Stella Polyhedron Navigator) 

Animation of folded form  Image of unfolded net form 
Screenshots indicating phases in SVG portion of animation sequence [click for separate SVG animation  rightclick on active animation for options  improvements welcome NB: Animation works in Firefox and Opera; effects do NOT work properly in Google Chrome, Internet Explorer or Safari] 

Early stage in animation  Late stage in animation 
Animation sequence subsequent to the SVG phase screenshots showing extremes of the folding (and unfolding) process on a green background (animation generated using Stella Polyhedron Navigator) 

Animation sequence subsequent to the SVG phase screenshots showing extremes of the folding (and unfolding) process on a green background (animation generated using Stella Polyhedron Navigator) 
The paper from which these animations were reproduced includes related commentaries on:
In discussion of the above illustration references was made to insights to be derived from the extensive literature on tiling, tesselation and the associated mathematics  most notably on a sphere. In mathematics, a spherical tiling or spherical polyhedron is a tiling of the sphere in which the surface is divided or partitioned by great arcs into bounded regions called spherical polygons.
As noted and illustrated by Wikipedia, the most familiar spherical polyhedron is the soccer ball (outside the USA and Australia, a football), thought of as a spherical truncated icosahedron. The Wikipedia entry offers a systematic array of such polyhedra.
All the regular, semiregular polyhedra and their duals can be projected onto the sphere as tilings. Given by their Schläfli symbol {p, q} or vertex figure a.b.c.
Examples of spherical tilings reproduced from Wikipedia  

Schläfli symbol  {p,q}  t{p,q}  r{p,q}  t{q,p}  {q,p}  rr{p,q}  tr{p,q}  sr{p,q} 
Vertex figure  pq  q.2p.2p  p.q.p.q  p.2q.2q  qp  q.4.p.4  4.2q.2p  3.3.q.3.p 
Tetrahedral (3 3 2) 
33 
3.6.6 
3.3.3.3 
3.6.6 
33 
3.4.3.4 
4.6.6 
3.3.3.3.3 
V3.6.6 
V3.3.3.3 
V3.6.6 
V3.4.4.4 
V4.6.6 
V3.3.3.3.3 

Octahedral (4 3 2) 
43 
3.8.8 
3.4.3.4 
4.6.6 
34 
3.4.4.4 
4.6.8 
3.3.3.3.4 
V3.8.8 
V3.4.3.4 
V4.6.6 
V3.4.4.4 
V4.6.8 
V3.3.3.3.4 

Icosahedral (5 3 2) 
53 
3.10.10 
3.5.3.5 
5.6.6 
35 
3.4.5.4 
4.6.10 
3.3.3.3.5 
V3.10.10 
V3.5.3.5 
V5.6.6 
V3.4.5.4 
V4.6.10 
V3.3.3.3.5 

Dihedral example p=6 (2 2 6) 
62 
2.12.12 
2.6.2.6 
6.4.4 
26 
4.6.4 
4.4.12 
3.3.3.6 
These images point to possible understandings of relationship links portrayed as curving arcs  rather than as straight lines, as is the tendency in network representations.
A particular feature of the approach is the use of forcedirected graph drawing, charactetistic of DataDriven Documents (d3.js), to elicit selforganizing convex polyhedra  without the conventional prerequisite for vertex coordinates (Elijah Meeks, D3.js in Action, 2015). A preliminary illustration of the approach was made in an earlier document (Optimizing Web Surfing Pathways for the Overloaded: polyhedral insights from the travelling salesman problem of operations research, 2015). This demonstrated the feasibility of eliciting a dodecahedron in this way  a classic example of a spherically symmetrical polyhedron.
As noted by Wikipedia, forcedirected methods in graph drawing date back to the work of W. T. Tutte (How to Draw a Graph, 1963), who showed that polyhedral graphs may be drawn in the plane with all faces convex by fixing the vertices of the outer face of a planar embedding of the graph into convex position, placing a springlike attractive force on each edge, and letting the system settle into an equilibrium. Because of the simple nature of the forces in this case, the system cannot get stuck in local minima, but rather converges to a unique global optimum configuration. Because of this work, embeddings of planar graphs with convex faces are sometimes called Tutte embeddings.
The concern here is to extend the dodecahedron in 3D to explore the possibility with the 13 semiregular Archimedean polyhedra  given newly demonstrated capacity of Tomá Fülöpp to detect triangles, squares, and other polygons in a large set of data using the algorithm of Robert E. Tarjan (Depthfirst search and linear graph algorithms. SIAM Journal on Computing, 1972).
The challenge was how to use the resource pool of disparate polygons to build polyhedra in the absence of the fixed coordinates normally required in define a polyhedron. The point of departure was the inherent connectivity of the polygons which implied the possibility of building larger patterns. The issue was how to use connectivity alone to enable the construction process. One problematic feature of such exploration is that polyhedral coordinates tend to be associated with applications which render the precise set of coordinates subject to copyright. Ironically this may also apply to connectvity data, but to a lesser degree.
The animations above illustrate the principles of the process. The further requirement is then obtaining the patterns of connectivity associated with a wide variety of polyhedra. This data is not readily available. It is typically implicit in formats required to construct such polyhedra for design purposes  as the manner in which points defined by coordnates are to be connected.
An extremely rich source of polyhedra of every kind is the Stella Polyhedron Navigator. One of the geometry definition export file formats it offers is .OFF. The approach taken here was therefore to export each of the Archimedean polyhedron from Stella. The connectivity data was extracted from each and reformatted as an adjacency list  namely a document constructed as an edge list of nodes (with names) and links in JSON format. An arbitrary collection of document labels was used in naming the nodes.
The JSON documents could then be visualized using the forcedirected layout facilities of d3.js, with results indicated below  after the dynamics had converged the connectivity into stable configurations. Preliminary issues with the dynamic displays resulted from a degree of incompetence with d3.js parameters (sizing the onscreen result, adjustment of the charge parameter, etc). The directional arrows and node colouring are again an arbitrary choice for purposes of illustration. Using the cursor in the dynamic displays does indeed bring up the (arbitrary) label of the node, but a hyperlink has not been activated on it. There is some question as to whether all browsers can handle that d3.js facility.
Screen shots of selected results of forcedirected layout to elicit Archimedean polyhedra (not to scale relative to one another; subject to further tests) 

Cuboctahedron (forcelayout animation) 
Small rhombicuboctahedron (forcelayout animation) 
Truncated cuboctahedron (forcelayout animation) 
Rotating animation from Wikipedia  Rotating animation from Wikipedia  Rotating animation from Wikipedia 
Access to separate interactive forcedirected animations of Archimedean polyhedra (visualization may be dependnt on browser and resizing)
Archimedean polyhedra (table adapted from original version in Wikipedia with added links to forcedirected animations ) 

Polyhedron  Transparent  Solid  Net  Faces  Edges  Vertices  
truncated tetrahedron [forcedirected version] 
(Animation) 
8  4 triangles 4 hexagons 
18  12  
cuboctahedron (rhombitetratetrahedron) [forcedirected version] 
(Animation) 
14  8 triangles 6 squares 
24  12  
truncated cube [forcedirected version] 
(Animation) 
14  8 triangles 6 octagons 
36  24  
truncated octahedron (truncated tetratetrahedron) [forcedirected version] 
14  6 squares 8 hexagons 
36  24  
rhombicuboctahedron (small rhombicuboctahedron) [forcedirected version] 
(Animation) 
26  8 triangles 18 squares 
48  24  
truncated cuboctahedron (great rhombicuboctahedron) [forcedirected version] 
(Animation) 
26  12 squares 8 hexagons 6 octagons 
72  48  
snub cube (snub cuboctahedron) [forcedirected version] 
(Animation) 
38  32 triangles 6 squares 
60  24  
icosidodecahedron [forcedirected version] 
(Animation) 
32  20 triangles 12 pentagons 
60  30  
truncated dodecahedron [forcedirected version] 
(Animation) 
32  20 triangles 12 decagons 
90  60  
truncated icosahedron [forcedirected version] 
(Animation) 
32  12 pentagons 20 hexagons 
90  60  
rhombicosidodecahedron (small rhombicosidodecahedron) [forcedirected version] 
(Animation) 
62  20 triangles 30 squares 12 pentagons 
120  60  
truncated icosidodecahedron (great rhombicosidodecahedron) [forcedirected version] 
(Animation) 
62  30 squares 20 hexagons 12 decagons 
180  120  
snub dodecahedron (snub icosidodecahedron) [forcedirected version] 
(Animation) 
92  80 triangles 12 pentagons 
150  60 
Of some concern was how d3.js would handle connectivity of unequal length  as is the case with the 2 or more edge types (lengths) in the semiregular Archimedean polyhedra, Further thought could be given to techniques for weighting the sizes in the converged result.
An interesting further possibility is to use the capacity of d3,js to log the coordinates of nodes of the converged result, as variously discussed (Stackoverflow, Airpair). The question would then be how these frozen coordinates might be exported and manipulated, possibly into a ,OFF format. This could then be imported into a mesh or computeraided design application (such as MeshLab, which uses a JSON format). Within that environment "irregularities" resulting from the forcelayout process might then be "normalized"  with additional graphics effects to enhance memorability
It is appropriate to respond to a potential criticism as to why not avoid the convolutions of the forcedirected approach by making direct use of known polyhedral coordinates. Whilst this might be a viable approach under some circumstances, the advantage of eliciting polygonal loops, as currently extracted, is that it does not preclude the identification of polyhedral forms which could be cumbersome to derive from data subject to intellectual copyright and proprietary formats.
There is every possibility that polyhedral forms based on much larger numbers of polygonal loops (and cycles) could be detected. The potential systemic significance of such structures merits distinct reflection where these are composed of:
Rather than the "carpet" metaphor used above, a more relevant metaphor may be associated with current recognition of the need for larger "tents" within which to gather together disparate entities. The metaphor is notably employed with respect to interfaith discourse which is currently proving to be so inadequate to the faithbased conflicts of the world. This argument is developed separately (Global Brane Comprehension Enabling a Higher Dimensional Big Tent? 2011). A charming irony noted there is the inspiration that tentmaking offered to the polymath Omar Khayyám  it being the traditional occupation of his family. Hence the relevance of "stitching" to the approach outlined above (Triangulation, connectivity and "stitching": enabling coherent global system dynamics, 2012).
Significantly missing from the above discussion is the sense of chirality by which polyhedral forms may be distinguished, namely as directed graphs there may be right and lefthanded variants of some of the forms above. This is increasingly important in molecular chemistry where the two forms may have quite distinct properties. As stressed, the arrows included into the forcedirected layouts above are there purely to indicate the need for such clarification.
Of particular interest is the relation between memorability and degrees of symmetry, as well as the memorability of some unusual structures with unusual forms of symmetry, such as the Szilassi polyhedron, as discussed separately (Mapping of WHquestions with questionpairs onto the Szilassi polyhedron, 2014).
Animations of Szilassi polyhedron of 7 faces (4 types), 21 edges (12 types), 14 vertices (7 types) [totalling 42=2x3x7; product=2x3^{2}7^{3}] (paired shapes coloured identically; prepared using Stella Polyhedron Navigator) 

Folding together of the two complementary nets 
Rotation of polyhedron  Screen shot of interactve forcedirected version 
Given the cyclic loops configured by this approach, of particular interest is the potential systemic significance of the polyhedral configurations which might emerge from the process outlined above. In this respect it is appropriate to recall the very extensive range of polyhedra of different degrees of symmetry, somce being convex and others concave. In the case of the drilled truncated cube (presented below), its use as a mapping framework is discussed separately (Changing Patterns using Transformation Pathways, 2015).
Drilled truncated cube of 32 faces (5 types), 64 edges (9 types), 32 vertices (4 types) [totalling 128=2^{7}] (animation prepared using Stella Polyhedron Navigator) 

Animation with faces nontransparent 
Screen shot of interactive forcedirected version 
AlbertLaszlo Barabasi. Linked: how everything is connected to everything else and what it means for business, science, and everyday life. 2014
Stephen P. Borgatti, Martin G. Everett and Jeffrey C. Johnson. Analyzing Social Networks. Sage, 2013
Lynn Cherny. Visualizing Networks: beyond the "hairball". O'Reilly Strata, 2012 [slides]
Nicholas A. Christakis and James H. Fowler. Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives. Back Bay Books, 2011
David Easley and Jon Kleinberg. Networks, Crowds, and Markets: reasoning about a highly connected world. Cambridge University Press, 2010
Tomá Fülöpp and Jacques de Mévius. Loop mining in the Encyclopedia of world problems. 2015 [text]
Paul Harrison. Robust topological sorting and Tarjan's algorithm in Python. Retrieved 9 February 2011. [text]
Charles Kadushin. Understanding Social Networks: theories, concepts, and findings. Oxford University Press, 2011
David Knoke and Song Yang. Social Network Analysis: quantitative applications in the social sciences. Sage, 2007
Elijah Meeks. D3.js in Action. Manning Publications, 2015
Arlind Nocaj, Mark Ortmann and Ulrik Brandes. Untangling Hairballs. Graph Drawing: Lecture Notes in Computer Science, 8871, 2014, pp 101112 [abstract]
Robert E. Tarjan. Depthfirst search and linear graph algorithms. SIAM Journal on Computing, 1, 1972, 2, pp. 146160 [doi]
W. T. Tutte. How to draw a graph. Proceedings of the London Mathematical Society, 13, 1963, 52, pp. 743768.
Duncan J. Watts. Six Degrees: the science of a connected age. W. W. Norton, 2004
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