Fundamental Domains

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

There are two algorithms for calculating fundamental domains in `HAPcryst`. 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.

a PolymakeObject
Let G be an AffineCrystGroupOnRight and v a vector. A fundamental domain containing v is calculated and returned as a PolymakeObject. The vector v is used as the starting point for a Dirichlet-Voronoi construction. If no v is given, the origin is used as starting point if it has trivial stabiliser. Otherwise an error is cast.
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
a PolymakeObject
Given a starting vector v and a Bieberbach group G in standard form, this method calculates the Dirichlet domain with respect to v. If gram is not supplied, the average gramian matrix is used (see ). It is not tested if gram is symmetric and positive definite. It is also not tested, if the product defined by gram is invariant under the point group of G.

The behaviour of this function is influenced by the option ineqThreshold ineqThreshold. 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 ineqThreshold, a new approximating polyhedron in calculated. The default for ineqThreshold is 200. Roughly speaking, a large threshold means shifting work from polymake to &GAP;, a small one means more calls of (and work for) polymake.

If the value of 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.

Note that the algorithm chooses vertices in random order and also writes inequalities for polymake in random order.
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>
a PolymakeObject
This uses an alternative algorithm based on geometric considerations. It is not used in any of the high-level methods. Let v be a vector and orbit a sufficiently large part of the orbit of v under a crystallographic group with standard- orthogonal point group (satisfying A^t=A^-1). A geometric algorithm is then used to calculate the Dirichlet domain with respect to v. This also works for crystallographic groups which are not Bieberbach. The point v has to have trivial stabilizer.

The intersection of the full orbit with the unit cube around v is sufficiently large.
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
true or false
This tests if a PolymakeObject poly is a fundamental domain for the affine crystallographic group G in standard form.

The function tests the following: First, does the orbit of any vertex of poly have a point inside poly (if this is the case, false is returned). Second: Is every facet of poly the image of a different facet under a group element which does not fix poly. If this is satisfied, true is returned.
true, false or fail
This tests if a PolymakeObject poly is a fundamental domain for the affine crystallographic group G in standard form and if this group is torsion free (ie a Bieberbach group)

It returns true if G is torsion free and poly is a fundamental domain for G. If poly is not a fundamental domain, false is returned regardless of the structure of G. And if G is not torsion free, the method returns fail.

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