Course MT4521 Geometry and topology

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## Exercises 6 1. The embedding of a regular tetrahedron in a cube shown on the right, gives a map from the full symmetry group of the tetrahedron: S(T) S4 to the full symmetry group of the cube: S(C) S4 × < J > .
Identify the image and kernel of this homomorphism.

2. Show that the 60 indirect symmetries of the dodecahedron consist of 15 reflections and 45 rotatory reflections.

3. Show that the groups C2 , C1 × < J > and C2 C1 are isomorphic but that they are not conjugate subgroups of I(R3).

4. Show that the group of rotations of a regular prism Pn = P × I R3 with P a regular polygon and I the unit interval, is the dihedral group Dn.
What is its full symmetry group ?

A square antiprism (with two parallel square faces and eight isosceles triangles for the others) is as shown on the right:
Find its direct and full symmetry groups.

5. Describe figures in R3 whose direct symmetry groups are each of the examples in the classification of rotational symmetry groups and which have no opposite symmetries.
[Hint: think about "painting designs" on the faces of some of the figures which do have opposite symmetries.]

Describe figures whose full symmetry groups are G × < J > with G each of the possible direct symmetry groups and where J is central inversion.

6. Describe a figure in R3 whose full symmetry group is the mixed group DnCn.
Find a figures whose full symmetry group is D2nDn.
Modify this last two example to find a figure with full symmetry group C2nCn.

7. Observe that one may "fill in the gap" between the upper halves of two regular octahedra with a regular tetrahedon and deduce that one may fill the whole of space with a mixture of regular octahedra and tetrahedra.

If you fill a large volume with this mixture, do you need more tetrahedra than octahedra, or less, or the same?

8. Figures are formed by placing two identical oblong tiles on top of each other with their centres in the same vertical line.
If the angles between their long axes are as shown:
(i) 0   (ii) π/2   (iii) π/3,
find the orders of their direct and full symmetry groups and identify these groups.

If the upper tile is coloured white and the lower one is coloured black, determine the direct and full symmetry groups in the three cases.

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JOC February 2010