fan:

====== Tutorial on hyperbolic surfaces ====== A //hyperbolic surface with cusps// is a topological surface together with a hyperbolic structure of finite area. ''%%polymake%%'' can deal with hyperbolic surfaces in view of Penners coordinates of the decorated Teichmüller space (lambda lengths). These allow to pick a hyperbolic surface by choosing a triangulation of the surface along with one positive parameter for each edge. The [[https://arxiv.org/abs/1708.08714v1|secondary fan]] of a hyperbolic surface stratifies the space of weight vectors (horocyclic decorations) according to which Delaunay triangulations are induced by the Epstein-Penner convex hull construction. For each point on the surface, there is a [[https://arxiv.org/abs/1708.08714v1|secondary polyhedron]] whose normal fan is the secondary fan. This tutorial shows how to deal with secondary fans and secondary polyhedra of hyperbolic surfaces. ==== Construction of hyperbolic surfaces ==== To define a hyperbolic surface we need to specify (a) a triangulation and (b) Penner coordinates. (a) The triangulation is obtained by specifying the ''%%DCEL%%'' as an ''%%Matrix%%''. This constructs a doubly connected edge list as follows: Each row of the nput of ''%%DCEL%%'' reads { (//2i//).head , (//2i+1//).head , (//2i//).next , (//2i+1//).next }. In general, for each edge //i// of the triangulation there are two half edges //2i// and //2i+1//, one for each orientation. (b) The ''%%PENNER_COORDINATES%%'' assign a positive rational number to each edge of the triangulation, ordered in the same sense as prescribed by the ''%%DCEL%%''. ==== Example 1: hyperbolic sphere with three cusps ====


> application 'fan';
> application 'topaz';
> $S3 = new Matrix([[1,0,2,5],[2,1,4,1],[0,2,0,3]]);
> $s = new HyperbolicSurface(DCEL=>$S3,PENNER_COORDINATES=>[1,1,1]);

In this example the doubly connected edge list looks like this:

==== The secondary fan ==== The secondary fan of the hyperbolic sphere from above can now be computed as follows.


> $f = $s->SECONDARY_FAN;
> $f->properties;
type: PolyhedralFan

MAXIMAL_CONES
{0 1 2}
{0 1 3}
{1 2 4}
{0 2 5}


RAYS
0 1 1
1 0 1
1 1 0
0 0 1
1 0 0
0 1 0

> $f->VISUAL;

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> $s->properties;
name: s
type: HyperbolicSurface

DCEL
N_vertices: 3
Halfedges list: (edgeIndex: twinIndex nextIndex prevIndex headIndex)
0: 1 2 4 1 (0 ---> 1)
1: 0 5 3 0 (1 ---> 0)
2: 3 4 0 2 (1 ---> 2)
3: 2 1 5 1 (2 ---> 1)
4: 5 0 2 0 (2 ---> 0)
5: 4 3 1 2 (0 ---> 2)


FLIP_WORDS
{}
{0}
{1}
{2}


PENNER_COORDINATES
1 1 1

SECONDARY_FAN
type: PolyhedralFan

The ''%%FLIP_WORDS%%'' indicate how to obtain the Delaunay triangulations. The k-th flip word is a list of integers (the indices of the edges) that describe which edge flips produce the k-th Delaunay triangulation. Note that the k-th Delaunay triangulation also corresponds to the k-th maximal cone of the ''%%SECONDARY_FAN%%''. ==== GKZ vectors & secondary polyhedra ==== In order to compute ''%%GKZ_VECTORS%%'' or a ''%%secondary_polyhedron%%'' of a hyperbolic surface one needs to additionally specify a ''%%SPECIAL_POINT%%'' on the surface. This is done by choosing two rational numbers. Continuing with the above example, lets look at the following.


> $s = new HyperbolicSurface(DCEL=>$S3,PENNER_COORDINATES=>[1,1,1],SPECIAL_POINT=>[1,0]);

Now we may compute an approximation of the ''%%GKZ_VECTORS%%'' of the surface. The approximation depends on a parameter //depth// that restricts the depth of the (covering) triangles that are summed over in the definition of the GKZ vectors.


> print $s->GKZ_VECTORS(3);
1 33346854621/25672050625 33346854621/25672050625 19782163/27238250
1 2361/3250 3955357/5447650 33346854621/25672050625
1 10549213550005124385885122/6365327663846199230365625 11433978/13287625 30327974429709/105771923977850
1 11433978/13287625 10549213550005124385885122/6365327663846199230365625 30327974429709/105771923977850

The secondary polyhedron can be computed similarly using the function ''%%secondary_polyhedron%%''.


> $p = secondary_polyhedron($s,10);
> $p->properties;
name: p
type: Polytope

CONE_AMBIENT_DIM
4

VERTICES
1 1.315301353 1.315301353 0.7316378744
1 0.7316489581 0.7316267908 1.315301353
1 1.752046187 0.8750928112 0.2910011302
1 0.8750928112 1.752046187 0.2910011302
0 -1 0 0
0 0 -1 0
0 0 0 -1


VERTICES_IN_FACETS
{0 1 3 4}
{0 1 2 5}
{0 2 3 6}
{1 4 5}
{2 5 6}
{3 4 6}

> $p->VISUAL(FacetColor=>'255 180 80');

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We may look at the GKZ domes of the individual Delaunay triangulations.


> $d0 = $s->gkz_dome(0,5);
> $d0->VISUAL(FacetColor=>'80 180 255');

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> $d1 = $s->gkz_dome(1,5);
> $d1->VISUAL(FacetColor=>'80 180 255');

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==== Example 2: a hyperbolic torus with three cusps ====


> $T3 = new Matrix([[1,0,2,17],[2,1,4,14],[0,2,0,6],[1,2,8,16],[0,1,5,10],[2,1,12,1],[0,2,9,3],[0,1,13,7],[0,2,15,11]]);
> $s = new HyperbolicSurface(DCEL=>$T3, PENNER_COORDINATES=>[2,1,1,1,1,1,1,1,1], SPECIAL_POINT=>[1,0]);


> $f = $s->SECONDARY_FAN;
> $f->VISUAL;

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> $s->properties;
name: s
type: HyperbolicSurface

DCEL
N_vertices: 3
Halfedges list: (edgeIndex: twinIndex nextIndex prevIndex headIndex)
0: 1 2 4 1 (0 ---> 1)
1: 0 17 11 0 (1 ---> 0)
2: 3 4 0 2 (1 ---> 2)
3: 2 14 13 1 (2 ---> 1)
4: 5 0 2 0 (2 ---> 0)
5: 4 6 8 2 (0 ---> 2)
6: 7 8 5 1 (2 ---> 1)
7: 6 16 15 2 (1 ---> 2)
8: 9 5 6 0 (1 ---> 0)
9: 8 10 12 1 (0 ---> 1)
10: 11 12 9 2 (1 ---> 2)
11: 10 1 17 1 (2 ---> 1)
12: 13 9 10 0 (2 ---> 0)
13: 12 3 14 2 (0 ---> 2)
14: 15 13 3 0 (1 ---> 0)
15: 14 7 16 1 (0 ---> 1)
16: 17 15 7 0 (2 ---> 0)
17: 16 11 1 2 (0 ---> 2)


FLIP_WORDS
{0}
{}
{0 3}
{0 4 7}
{0 6}
{3}
{6}
{0 3 1 5}
{0 6 2 8}
{3 1 5}
{6 2 8}
{0 3 1 5 0 1}
{0 6 2 8 0 2}


PENNER_COORDINATES
2 1 1 1 1 1 1 1 1

SECONDARY_FAN
type: PolyhedralFan

SPECIAL_POINT
1 0
> $p = secondary_polyhedron($s,7);
> $p->VISUAL(FacetColor=>'255 180 80');

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> $d0 = $s->gkz_dome(0,5);
> $d0->VISUAL(FacetColor=>'80 180 255');

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> $s = new HyperbolicSurface(DCEL=>$T3, PENNER_COORDINATES=>[2,1,1,1,1,1,1,1,1], SPECIAL_POINT=>[new Rational(1.5196714),new Rational(-0.5773503)]);
> $p = secondary_polyhedron($s,7);
> $p->VISUAL(FacetColor=>'255 180 80');

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> $d0 = $s->gkz_dome(0,5);
> $d0->VISUAL(FacetColor=>'80 180 255');

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==== More examples can be studied via the following: ====


> # a torus with two cusps (6 edges)
> $T2 = new Matrix([[0,0,6,5],[0,0,1,10],[0,0,8,2],[1,0,11,4],[1,0,7,3],[1,0,9,0]]);
> 
> # a sphere with four cusps (6 edges)
> $S4 = new Matrix([[1,0,2,6],[2,1,4,9],[0,2,0,11],[3,0,8,5],[1,3,1,10],[2,3,3,7]]);
> 
> # a double torus with two cusps (12 edges)
> $DT2 = new Matrix([[0,0,8,10],[0,0,12,14],[0,0,16,18],[0,0,20,22],[1,0,23,2],[1,0,13,3],[1,0,9,1],[1,0,11,4],[1,0,15,6],[1,0,21,7],[1,0,17,5],[1,0,19,0]]);

To study 4-dim. secondary fans the following method is useful. It intersects the secondary fan with the 3-dim. standard simplex.


> sub norm($){
>    my $B = new Matrix(shift);
>    for (my $i = 0; $i < $B->rows(); ++$i) {
>       my $sum = 0;
>       for (my $j = 1; $j < $B->cols(); ++$j) {
>          $sum = $sum + $B->elem($i,$j);
>       }
>       $x = 1/$sum;
>       $B->row($i) = $x * $B->row($i);
>    }
>    return $B;
> }
> $s = new HyperbolicSurface(DCEL=>$S4,PENNER_COORDINATES=>[1,1,1,1,1,1],SPECIAL_POINT=>[1,0]);
> $f = $s->SECONDARY_FAN;
> $v = ones_vector | $f->RAYS;
> $a = norm($v);
> $b = $a->minor(All,~[0]);
> $c = ones_vector | $b;
> $q = new fan::PolyhedralComplex(POINTS=>$c,INPUT_POLYTOPES=>rows($f->MAXIMAL_CONES));
> $pro = fan::project_full($q);
> $pro->VISUAL;

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