fundamentaldomain.xml

<Section><Heading>Fundamental Domains</Heading>

Let <M>S</M> be a crystallographic group. A Fundamental domain is a closed
convex set containing a system of representatives for the Orbits of <M>S</M> in
its natural action on euclidian space.<Br/>

There are two algorithms for calculating fundamental domains in
<Package>HAPcryst</Package>. One uses the geometry and relies on having the
standard rule for evaluating the scalar product (i.e. the gramian matrix is the
identity). The other one is independent of the gramian matrix but does only
work for Bieberbach groups, while the first ("geometric") algorithm works for
arbitrary crystallographic groups given a point with trivial stabilizer.

<ManSection>
<Meth Name="FundamentalDomainStandardSpaceGroup" Arg="[v], G" />
<Meth Name="FundamentalDomainStandardSpaceGroup" Arg="v, G" />
 <Returns>a <K>PolymakeObject</K></Returns>
  <Description>
   Let <A>G</A> be an <K>AffineCrystGroupOnRight</K> and <A>v</A> a vector. A
   fundamental domain containing <A>v</A> is calculated and returned as a
   <K>PolymakeObject</K>. The vector <A>v</A> is used as the starting point for
   a Dirichlet-Voronoi construction. If no <A>v</A> is given, the origin is
   used as starting point if it has trivial stabiliser. Otherwise an error is 
   cast. <Br />
  </Description>
</ManSection>

<Example><![CDATA[
gap> fd:=FundamentalDomainStandardSpaceGroup([1/2,0,1/5],SpaceGroup(3,9));
<polymake object>
gap> Polymake(fd,"N_VERTICES");
24
gap> fd:=FundamentalDomainStandardSpaceGroup(SpaceGroup(3,9));
<polymake object>
gap> Polymake(fd,"N_VERTICES");
8
]]></Example>

<ManSection>
<Meth Name="FundamentalDomainBieberbachGroup" Arg="G"/> 
<Meth Name="FundamentalDomainBieberbachGroup" Arg="v G [gram]"/> 
 <Returns>a <K>PolymakeObject</K></Returns>
 <Description>
  Given a starting vector <A>v</A> and a Bieberbach group <A>G</A> in
  standard form, this method calculates the Dirichlet domain with
  respect to <A>v</A>. If <A>gram</A> is not supplied, the average
  gramian matrix is used (see
  <Ref Meth="GramianOfAverageScalarProductFromFiniteMatrixGroup"/>).
  It is not tested if <A>gram</A> is symmetric and positive
  definite. It is also not tested, if the product defined
  by <A>gram</A> is invariant under the point group of <A>G</A>. 
  <P/>

  The behaviour of this function is influenced by the option 
  <K>ineqThreshold</K><Label
  Name="ineqThreshold"/><Index>ineqThreshold</Index>. 
  The algorithm calculates
  approximations to a fundamental domain by iteratively adding inequalities.
  For an approximating polyhedron, every vertex is tested to find new
  inequalities. When all vertices have been considered or the number of new 
  inequalities already found exceeds
  the value of <K>ineqThreshold</K>, a new approximating polyhedron in
  calculated. The default for <K>ineqThreshold</K> is 200. Roughly speaking,
  a large threshold means shifting work from <K>polymake</K> to &GAP;, a small
  one means more calls of (and work for) <K>polymake</K>.
  <P/>
  If the value of <Ref InfoClass="InfoHAPcryst"/> is 2 or more, for each
  approximation the number of vertices of the approximation, the number of
  vertices that have to be considered during the calculation, the number of
  facets, and  new inequalities is shown.
  <P/>
 
  Note that the algorithm chooses vertices in random order and also writes
  inequalities for <K>polymake</K> in random order.
 </Description>
</ManSection>


<Example><![CDATA[
gap> a0:=[[ 1, 0, 0, 0, 0, 0, 0 ], [ 0, -1, 0, 0, 0, 0, 0 ], 
>     [ 0, 0, 1, 0, 0, 0, 0 ], [ 0, 0, 0, 1, 0, 0, 0 ], 
>     [ 0, 0, 0, 0, 0, 1, 0 ], [ 0, 0, 0, 0, -1, -1, 0 ],
>     [ -1/2, 0, 0, 1/6, 0, 0, 1 ] 
>     ];;
gap> a1:=[[ 0, -1, 0, 0, 0, 0, 0 ],[ 0, 0, -1, 0, 0, 0, 0 ],
>         [ 1, 0, 0, 0, 0, 0, 0 ], [ 0, 0, 0, 1, 0, 0, 0 ], 
>         [ 0, 0, 0, 0, 1, 0, 0 ], [ 0, 0, 0, 0, 0, 1, 0 ],
>         [ 0, 0, 0, 0, 1/3, -1/3, 1 ] 
>        ];;
gap> trans:=List(Group((1,2,3,4,5,6)),g->
>           TranslationOnRightFromVector(Permuted([1,0,0,0,0,0],g)));;
gap> S:=AffineCrystGroupOnRight(Concatenation(trans,[a0,a1]));
<matrix group with 8 generators>
gap> SetInfoLevel(InfoHAPcryst,2);
gap> FundamentalDomainBieberbachGroup(S:ineqThreshold:=10);
#I  v: 104/104 f:15
#I  new: 201
#I  v: 961/961 f:58
#I  new: 20
#I  v: 1143/805 f:69
#I  new: 12
#I  v: 1059/555 f:64
#I  new: 15
#I  v: 328/109 f:33
#I  new: 12
#I  v: 336/58 f:32
#I  new: 0
<polymake object>
gap> FundamentalDomainBieberbachGroup(S:ineqThreshold:=1000);
#I  v: 104/104 f:15
#I  new: 149
#I  v: 635/635 f:41
#I  new: 115
#I  v: 336/183 f:32
#I  new: 0
#I  out of inequalities
<polymake object>]]>
</Example>

<ManSection>
<Meth Name="FundamentalDomainFromGeneralPointAndOrbitPartGeometric" Arg="v
orbit"/>
 <Returns>a <K>PolymakeObject</K></Returns>
 <Description>
  This uses an alternative algorithm based on geometric considerations. It is 
  not used in any of the high-level methods.
  Let <A>v</A> be a vector and <A>orbit</A> a sufficiently large part of the
  orbit of <A>v</A> under a crystallographic group with standard- orthogonal
  point group (satisfying <M>A^t=A^-1</M>). A geometric algorithm is then used
  to calculate the Dirichlet domain with respect to <A>v</A>. This also works
  for crystallographic groups which are not Bieberbach. The point <A>v</A> has
  to have trivial stabilizer.<Br/>

  The intersection of the full orbit with the unit cube around <A>v</A> is
  sufficiently large.
 </Description>
</ManSection>

<Example><![CDATA[
gap> G:=SpaceGroup(3,9);;
gap> v:=[0,0,0];
[ 0, 0, 0 ]
gap> orbit:=OrbitStabilizerInUnitCubeOnRight(G,v).orbit;
[ [ 0, 0, 0 ], [ 0, 0, 1/2 ] ]
gap> fd:=FundamentalDomainFromGeneralPointAndOrbitPartGeometric(v,orbit);
<polymake object>
gap> Polymake(fd,"N_VERTICES");
8
]]></Example>


<ManSection>
  <Meth Name="IsFundamentalDomainStandardSpaceGroup"
        Arg="poly G"/>
  <Returns>
   true or false
  </Returns>
  <Description>
   This tests if a <K>PolymakeObject</K> <A>poly</A> is a fundamental domain
   for the affine crystallographic group <A>G</A> in standard form.<Br/>
   
   The function tests the following: First, does the orbit of any vertex
   of <A>poly</A> have a point inside <A>poly</A> (if this is the
   case, <C>false</C> is returned).  Second: Is every facet
   of <A>poly</A> the image of a different facet under a group element
   which does not fix <A>poly</A>. If this is satisfied,
   <C>true</C> is returned.
  </Description>
</ManSection>


<ManSection>
  <Meth Name="IsFundamentalDomainBieberbachGroup"
        Arg="poly G"/>
  <Returns>
   true, false or fail
  </Returns>
  <Description>
   This tests if a <K>PolymakeObject</K> <A>poly</A> is a fundamental domain
   for the affine crystallographic group <A>G</A> in standard form and if this
   group is torsion free (ie a Bieberbach group)<Br />
   
   It returns <K>true</K> if <A>G</A> is torsion free and <A>poly</A> is a
   fundamental domain for <A>G</A>. If <A>poly</A> is not a fundamental domain,
   <K>false</K> is returned regardless of the structure of <A>G</A>. And if
   <A>G</A> is not torsion free, the method returns <K>fail</K>.

   If <A>G</A> is polycyclic, torsion freeness is tested using a representation
   as pcp group. Otherwise the stabilisers of the faces of the fundamental
   domain <A>poly</A> are calculated (<A>G</A> is torsion free if and only 
   if it all these stabilisers are trivial).
  </Description>
</ManSection>




</Section>