Octonion Multiplication Tables

If octonion units u, v, w are a quaternionic triple
of octonions (in that order), then by definition

uv = -vu = w, and vw = - wv = u, and wu = - uw = v.

There are 4 octonion multiplication tables for which index cycling

(e_{a}e_{b} = e_{c} -->
e_{a+1}e_{b+1} = e_{c+1})
and doubling
(e_{a}e_{b} = e_{c} -->
e_{2a}e_{2b} = e_{2c})

are automorphisms. Their quaternionic triples, hence their multiplication tables,

are determined by the following diagrams:

C(+) Table: | e_{1} |
e_{2} |
e_{3} |
e_{4} |
e_{5} |
e_{6} |
e_{7} |
....... | D(+) Table: | e_{1} |
e_{2} |
e_{3} |
e_{4} |
e_{5} |
e_{6} |
e_{7} |

D(-) Table: | e_{7} |
e_{6} |
e_{5} |
e_{4} |
e_{3} |
e_{2} |
e_{1} |
....... | C(-) Table: | e_{7} |
e_{6} |
e_{5} |
e_{4} |
e_{3} |
e_{2} |
e_{1} |

C(-) Table: | -e_{1} |
-e_{2} |
-e_{3} |
-e_{4} |
-e_{5} |
-e_{6} |
-e_{7} |
....... | D(-) Table: | -e_{1} |
-e_{2} |
-e_{3} |
-e_{4} |
-e_{5} |
-e_{6} |
-e_{7} |

D(+) Table: | -e_{7} |
-e_{6} |
-e_{5} |
-e_{4} |
-e_{3} |
-e_{2} |
-e_{1} |
....... | C(+) Table: | -e_{7} |
-e_{6} |
-e_{5} |
-e_{4} |
-e_{3} |
-e_{2} |
-e_{1} |

In each case the highlighted units determine an ordered quaternionic triple, and

the rest of the triples can be determined by sliding the highlighted pattern cyclicly

through 7 possible positions. The D(-) table is determined from the C(+) table by

reversing the order of the e

same is true of the D(+) and C(-) tables.

As indicated, however, we can go from the C(+) (or D(+)) table to the C(-) (or D(-)) table

by maintaining the order of indices and changing the signs of the octonion units in the diagram.

This is because in general if e

so are -e

And finally, as indicated, one can go from the C(+) (or D(+)) table to the D(+) (or C(+))

tables by changing all signs and reversing the order of the indices (an odd number of sign

changes, and an odd number of permutations).

There are 480 octonion multiplication tables for which e

for all indices a and b, and such that the identity is represented by the

unit e

and sign changes of the units appearing in the C(+) diagram above (top left diagram).

There are (7!)(2

a redundancy of

645120 / 480 = 1344.

We can get to

- 120 distinct tables with an

**even**number of sign changes, and an**even**number or permutations

(this will include C(+)); - 120 distinct tables with an

**odd**number of sign changes, and an**even**number or permutations

(this will include C(-)); - 120 distinct tables with an

**even**number of sign changes, and an**odd**number or permutations

(this will include D(-)); - 120 distinct tables with an

**odd**number of sign changes, and an**odd**number or permutations

(this will include D(+)).

Performing only sign changes without permutations doesn't get us very far. There

is a redundancy of order 8 associated with sign changes. For example, there are

8 sign changing automorphisms: the identity map is one; and changing the signs of

all units save some quaternionic triple give 7 others. The map on C(+) that takes

e

is an automorphism and so has no effect on the multiplication table.

The other 6 nontrivial sign-changing automorphisms can be got by cycling through

the indices (this is one very nice feature of this index cycling automorphism:

specific results on the units immediately become quite general).

These automorphisms arise from an even number of sign changes, of which there are

are 2

tables that arise from an even number of sign changes (all of which are accessible

via permutations alone), and 8 multiplication tables that arise from an odd number

of sign changes. These multiplication tables are associated with special X-products

(to be defined on Page 4).

Page 2. | Contents | Octoshop III Page.