Skeletal Graphs of Triply Periodic Surfaces
Although the notion of a skeletal graph of a surface does not have a
precise mathematical definition, several experiments suggest that
certain surfaces do have unique skeletal graphs.
The skeletal graph can be thought of as the end result of expanding
or shrinking a surface along the direction of
its normal vectors, while avoiding any pinching off that would change
the topology of the surface, until all that remains is a connected
graph of arcs and nodes.
Examples of surfaces for which there are obvious skeletal graphs are
the triply periodic P, D, G, and iWp surfaces.
These graphs are identical whatever the 'flavor' (minimal, CMC, or level)
of the surface.
Here are skeletal graphs for all of the surfaces listed on the
Triply Periodic Level Surfaces page,
where the thickened representations of the skeletal graphus were
generated using level set methods.
These surfaces, being oriented, have two skeletal graphs, which are
designated primary and secondary.
The primary and secondary graphs of the P and D surfaces are congruent;
those of the G surface have mirror symmetry with eachother;
and the primary and secondary graphs of the W and N surfaces are different.
The domain of 2 pi (meaning the cube enclosing the volume from pi to pi
in X, Y, and Z) gives the unit cell for all the surfaces.
The unit cell is the smallest portion of space which can be repeated in
the X, Y, and Z directions with translations to form the complete surface.
For the statistics, the unit cell's dimensions are normalized to occupy a
cube whose sides are length 2.
(NOTE: The secondary skeletal graph for the 'W' (iWp) surface is incorrect.)
Controls 
Key 
Display 
Miller Indices 
Domain 





: length of arcs
: number of arcs per unit cell
: total arclength per unit cell
: number of arcs meeting at nodes
: number of nodes per unit cell
: surface area in unit cell


surface 
view 
statistics 
G  

D  

P  

W  
  1.732; 0.707 
  8; 24 
  13.86; 16.97 
  8; 4 
  2; 12 
  14.20 

N  
  1.414; 1.414 
  12; 12 
  16.97; 16.97 
  12,4; 6,4 
  1,3; 1,3 
  14.13 

The thichened representations of skeletal graphis depicted here were
generated using as level surfaces of some rather complicated
expressions.
A much simpler approach to approximating the same skeletal graph also using
level surfaces, illustrated
here.