So far, the puzzles are attractive but tedious. A trailer is in front of the Transformers movie, I've heard. Puzzle Picnic Johan de Ruiter:

Posted on August 4, by apgoucher In three dimensions, the familiar cross product is a bilinear function expressible in terms of the Levi-Civita alternating tensor. It should be evident that for orthogonality and isotropy, the cross product can only be defined in three dimensions.

In two dimensions or lower, there are insufficiently many dimensions for orthogonality. On the other hand, in four or more dimensions, we can apply a rotation that preserves b and c whilst changing a, so the cross product cannot be well defined.

Hence, it only exists in three dimensions unless it is identically zero, which is stupid and trivial. However, if we relax the condition of total isotropy, there exists a seven-dimensional analogue of the cross product. It is uniquely determined up to automorphism of the underlying vector space by the following conditions: Bilinearity; Complete antisymmetry; Orthogonality.

The automorphism group is the compact real form of the exceptional Lie group G2. The projection shown above highlights the antisymmetry of the tensor — reflecting in an axis causes the colours of the entries to alternate. Note that the position x, y, z is non-zero if and only if x, y and z are collinear on a particular labelling of the Fano plane.

Then, the rule for multiplying quaternions becomes: Using a seven-dimensional cross product, we obtain the rule for multiplying weird eight-dimensional entities called octonions. Split octonions The squared norm of an octonion is defined to be the sum of the squares of its components, just as one would define the norm of a complex number.

Equivalently, we can consider it to be the length of an octonion as a vector in eight-dimensional space. When the seven-dimensional subspace of purely imaginary octonions is projectivised the origin is thrown away and real scalar multiples are associatedwe get a beautiful six-dimensional projective space.

John Baez and Huerta discovered that this is precisely the space of positions of a spinorial ball rolling on the surface of a projective ball of three times its diameter. And the symmetry group of this six-dimensional projective space? The split form of G2, naturally….The tetrahedron is self-dual.

The regular dodecahedron is obtained from a cube by constructing “roofs” on the faces of the cube (Euclid’s method). Any four vertices of a cube that are not pairwise incident on an edge are the vertices of a tetrahedron. To change a decimal to a fraction, write in the denominator a 1 followed by as many zeroes as there are digits to the right of the decimal point, and write in the numerator all the digits in the original decimal, omitting the decimal point.

In Discover Magazine, the latest Jaron's World column talks about the cell, a four-dimensional self-dual abstract regular polytope. As a graph, it's K11, the complete graph on 11 vertices.

As a graph, it's K11, the complete graph on 11 vertices. The regular tetrahedron is self-dual, Especially in roleplaying, this solid is known as a 4-sided die, one of the more common polyhedral dice, with the number rolled appearing around the bottom or on the top vertex.

Some Rubik's Diameter: 1.

Polyhedral dice shaped like the tetrakis hexahedron are matrix inversion. This is the subgroup of the general linear group given by the kernel of the determinant det. Where we write F× for the group of F. to the bicylinder Steinmetz solid, the dual of the n-gonal hosohedron is the n-gonal dihedron.

The polyhedron is self-dual, and is. A PL-map between two polyhedral spaces is a continuous map which, for certain triangulation of the spaces maps every simplex from the first space to a simplex in the target space in a linear fashion.

It has been known due to V. Zalgaller that any 2 or 3-dimensional polyhedral space admits a PL-isometry into \Bbb R^2 (\Bbb R^3 .

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Octahedral symmetry - Wikipedia