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### 3 Algorithms of Orbit-Stabilizer Type

We introduce a way to calculate a sufficient part of an orbit and the stabilizer of a point.

#### 3.1 Orbit Stabilizer for Crystallographic Groups

##### 3.1-1 OrbitStabilizerInUnitCubeOnRight
 `> OrbitStabilizerInUnitCubeOnRight`( group, x ) ( method )

Returns: A record containing

• `.stabilizer`: the stabilizer of x.

• `.orbit` set of vectors from [0,1)^n which represents the orbit.

Let x be a rational vector from [0,1)^n and group a space group in standard form. The function then calculates the part of the orbit which lies inside the cube [0,1)^n and the stabilizer of x. Observe that every element of the full orbit differs from a point in the returned orbit only by a pure translation.

Note that the restriction to points from [0,1)^n makes sense if orbits should be compared and the vector passed to `OrbitStabilizerInUnitCubeOnRight` should be an element of the returned orbit (part).

 ``` gap> S:=SpaceGroup(3,5);; gap> OrbitStabilizerInUnitCubeOnRight(S,[1/2,0,9/11]); rec( orbit := [ [ 0, 1/2, 2/11 ], [ 1/2, 0, 9/11 ] ], stabilizer := Group([ [ [ 1, 0, 0, 0 ], [ 0, 1, 0, 0 ], [ 0, 0, 1, 0 ], [ 0, 0, 0, 1 ] ] ]) ) gap> OrbitStabilizerInUnitCubeOnRight(S,[0,0,0]); rec( orbit := [ [ 0, 0, 0 ] ], stabilizer := ) ```

If you are interested in other parts of the orbit, you can use `VectorModOne` (2.1-2) for the base point and the functions `ShiftedOrbitPart` (3.1-9), `TranslationsToOneCubeAroundCenter` (3.1-10) and `TranslationsToBox` (3.1-11) for the resulting orbit
Suppose we want to calculate the part of the orbit of `[4/3,5/3,7/3]` in the cube of sidelength `1` around this point:

 ```gap> S:=SpaceGroup(3,5);; gap> p:=[4/3,5/3,7/3];; gap> o:=OrbitStabilizerInUnitCubeOnRight(S,VectorModOne(p)).orbit; [ [ 1/3, 2/3, 1/3 ], [ 1/3, 2/3, 2/3 ] ] gap> box:=p+[[-1,1],[-1,1],[-1,1]]; [ [ 1/3, 8/3, 7/3 ], [ 1/3, 8/3, 7/3 ], [ 1/3, 8/3, 7/3 ] ] gap> o2:=Concatenation(List(o,i->i+TranslationsToBox(i,box)));; gap> # This is what we looked for. But it is somewhat large: gap> Size(o2); 54 ```

##### 3.1-2 OrbitStabilizerInUnitCubeOnRightOnSets
 `> OrbitStabilizerInUnitCubeOnRightOnSets`( group, set ) ( method )

Returns: A record containing

• `.stabilizer`: the stabilizer of set.

• `.orbit` set of sets of vectors from [0,1)^n which represents the orbit.

Calculates orbit and stabilizer of a set of vectors. Just as `OrbitStabilizerInUnitCubeOnRight` (3.1-1), it needs input from [0,1)^n. The returned orbit part `.orbit` is a set of sets such that every element of `.orbit` has a non-trivial intersection with the cube [0,1)^n. In general, these sets will not lie inside [0,1)^n completely.

 ```gap> S:=SpaceGroup(3,5);; gap> OrbitStabilizerInUnitCubeOnRightOnSets(S,[[0,0,0],[0,1/2,0]]); rec( orbit := [ [ [ -1/2, 0, 0 ], [ 0, 0, 0 ] ], [ [ 0, 0, 0 ], [ 0, 1/2, 0 ] ], [ [ 1/2, 0, 0 ], [ 1, 0, 0 ] ] ], stabilizer := Group([ [ [ 1, 0, 0, 0 ], [ 0, 1, 0, 0 ], [ 0, 0, 1, 0 ], [ 0, 0, 0, 1 ] ] ]) ) ```

##### 3.1-3 OrbitPartInVertexSetsStandardSpaceGroup
 `> OrbitPartInVertexSetsStandardSpaceGroup`( group, vertexset, allvertices ) ( method )

Returns: Set of subsets of allvertices.

If allvertices is a set of vectors and vertexset is a subset thereof, then `OrbitPartInVertexSetsStandardSpaceGroup` returns that part of the orbit of vertexset which consists entirely of subsets of allvertices. Note that,unlike the other `OrbitStabilizer` algorithms, this does not require the input to lie in some particular part of the space.

 ```gap> S:=SpaceGroup(3,5);; gap> OrbitPartInVertexSetsStandardSpaceGroup(S,[[0,1,5],[1,2,0]], > Set([[1,2,0],[2,3,1],[1,2,6],[1,1,0],[0,1,5],[3/5,7,12],[1/17,6,1/2]])); [ [ [ 0, 1, 5 ], [ 1, 2, 0 ] ], [ [ 1, 2, 6 ], [ 2, 3, 1 ] ] ] gap> OrbitPartInVertexSetsStandardSpaceGroup(S, [[1,2,0]], > Set([[1,2,0],[2,3,1],[1,2,6],[1,1,0],[0,1,5],[3/5,7,12],[1/17,6,1/2]])); [ [ [ 0, 1, 5 ] ], [ [ 1, 1, 0 ] ], [ [ 1, 2, 0 ] ], [ [ 1, 2, 6 ] ], [ [ 2, 3, 1 ] ] ] ```

##### 3.1-4 OrbitPartInFacesStandardSpaceGroup
 `> OrbitPartInFacesStandardSpaceGroup`( group, vertexset, faceset ) ( method )

Returns: Set of subsets of faceset.

This calculates the orbit of a space group on sets restricted to a set of faces.
If faceset is a set of sets of vectors and vertexset is an element of faceset, then `OrbitPartInFacesStandardSpaceGroup` returns that part of the orbit of vertexset which consists entirely of elements of faceset.
Note that,unlike the other `OrbitStabilizer` algorithms, this does not require the input to lie in some particular part of the space.

##### 3.1-5 OrbitPartAndRepresentativesInFacesStandardSpaceGroup
 `> OrbitPartAndRepresentativesInFacesStandardSpaceGroup`( group, vertexset, faceset ) ( method )

Returns: A set of face-matrix pairs .

This is a slight variation of `OrbitPartInFacesStandardSpaceGroup` (3.1-4) that also returns a representative for every orbit element.

 ```gap> S:=SpaceGroup(3,5);; gap> OrbitPartInVertexSetsStandardSpaceGroup(S,[[0,1,5],[1,2,0]], > Set([[1,2,0],[2,3,1],[1,2,6],[1,1,0],[0,1,5],[3/5,7,12],[1/17,6,1/2]])); [ [ [ 0, 1, 5 ], [ 1, 2, 0 ] ], [ [ 1, 2, 6 ], [ 2, 3, 1 ] ] ] gap> OrbitPartInFacesStandardSpaceGroup(S,[[0,1,5],[1,2,0]], > Set( [ [ [ 0, 1, 5 ], [ 1, 2, 0 ] ], [[1/17,6,1/2],[1,2,7]]])); [ [ [ 0, 1, 5 ], [ 1, 2, 0 ] ] ] gap> OrbitPartAndRepresentativesInFacesStandardSpaceGroup(S,[[0,1,5],[1,2,0]], > Set( [ [ [ 0, 1, 5 ], [ 1, 2, 0 ] ], [[1/17,6,1/2],[1,2,7]]])); [ [ [ [ 0, 1, 5 ], [ 1, 2, 0 ] ], [ [ 1, 0, 0, 0 ], [ 0, 1, 0, 0 ], [ 0, 0, 1, 0 ], [ 0, 0, 0, 1 ] ] ] ] ```

##### 3.1-6 StabilizerOnSetsStandardSpaceGroup
 `> StabilizerOnSetsStandardSpaceGroup`( group, set ) ( method )

Returns: finite group of affine matrices (OnRight)

Given a set set of vectors and a space group group in standard form, this method calculates the stabilizer of that set in the full crystallographic group.

 ``` gap> G:=SpaceGroup(3,12);; gap> v:=[ 0, 0,0 ];; gap> s:=StabilizerOnSetsStandardSpaceGroup(G,[v]); gap> s2:=OrbitStabilizerInUnitCubeOnRight(G,v).stabilizer; gap> s2=s; true ```

##### 3.1-7 RepresentativeActionOnRightOnSets
 `> RepresentativeActionOnRightOnSets`( group, set, imageset ) ( method )

Returns: Affine matrix.

Returns an element of the space group S which takes the set set to the set imageset. The group must be in standard form and act on the right.

 ```gap> S:=SpaceGroup(3,5);; gap> RepresentativeActionOnRightOnSets(G, [[0,0,0],[0,1/2,0]], > [ [ 0, 1/2, 0 ], [ 0, 1, 0 ] ]); [ [ 0, -1, 0, 0 ], [ -1, 0, 0, 0 ], [ 0, 0, -1, 0 ], [ 0, 1, 0, 1 ] ] ```

##### 3.1-8 Getting other orbit parts

HAPcryst does not calculate the full orbit but only the part of it having coefficients between -1/2 and 1/2. The other parts of the orbit can be calculated using the following functions.

##### 3.1-9 ShiftedOrbitPart
 `> ShiftedOrbitPart`( point, orbitpart ) ( method )

Returns: Set of vectors

Takes each vector in orbitpart to the cube unit cube centered in point.

 ```gap> ShiftedOrbitPart([0,0,0],[[1/2,1/2,1/3],-[1/2,1/2,1/2],[19,3,1]]); [ [ 1/2, 1/2, 1/3 ], [ 1/2, 1/2, 1/2 ], [ 0, 0, 0 ] ] gap> ShiftedOrbitPart([1,1,1],[[1/2,1/2,1/2],-[1/2,1/2,1/2]]); [ [ 3/2, 3/2, 3/2 ] ] ```

##### 3.1-10 TranslationsToOneCubeAroundCenter
 `> TranslationsToOneCubeAroundCenter`( point, center ) ( method )

Returns: List of integer vectors

This method returns the list of all integer vectors which translate point into the box center+[-1/2,1/2]^n

 ```gap> TranslationsToOneCubeAroundCenter([1/2,1/2,1/3],[0,0,0]); [ [ 0, 0, 0 ], [ 0, -1, 0 ], [ -1, 0, 0 ], [ -1, -1, 0 ] ] gap> TranslationsToOneCubeAroundCenter([1,0,1],[0,0,0]); [ [ -1, 0, -1 ] ] ```

##### 3.1-11 TranslationsToBox
 `> TranslationsToBox`( point, box ) ( method )

Returns: An iterator of integer vectors or the empty iterator

Given a vector v and a list of pairs, this function returns the translation vectors (integer vectors) which take v into the box box. The box box has to be given as a list of pairs.

 ```gap> TranslationsToBox([0,0],[[1/2,2/3],[1/2,2/3]]); [ ] gap> TranslationsToBox([0,0],[[-3/2,1/2],[1,4/3]]); [ [ -1, 1 ], [ 0, 1 ] ] gap> TranslationsToBox([0,0],[[-3/2,1/2],[2,1]]); Error, Box must not be empty called from ... ```
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