Log24

AN INVARIANCE OF SYMMETRY

BY STEVEN H. CULLINANE

We present a simple, surprising, and beautiful combinatorial
invariance of geometric symmetry, in an algebraic setting.

DEFINITION. A delta transform of a square array over a 4-set is
any pattern obtained from the array by a 1-to-1 substitution of the
four diagonally-divided two-color unit squares for the 4-set elements.

THEOREM. Every delta transform of the Klein group table has
ordinary or color-interchange symmetry, and remains symmetric under
the group G of 322,560 transformations generated by combining
permutations of rows and colums with permutations of quadrants.

PROOF (Sketch). The Klein group is the additive group of GF (4);
this suggests we regard the group's table  T as a matrix over that
field. So regarded, T is a linear combination of three (0,1)-matrices
that indicate the locations, in  T, of the 2-subsets of field elements.
The structural symmetry of these matrices accounts for the symmetry
of the delta transforms of  T, and is invariant under G.

All delta transforms of the 4⁵ matrices in the algebra generated by
the images of  T under G are symmetric; there are many such algebras. 

THEOREM. If 1 m ≤ n²+2, there is an algebra of 4^m
2n x 2n matrices over GF(4) with all delta transforms symmetric.

An induction proof constructs sets of basis matrices that yield
the desired symmetry and ensure closure under multiplication.

REFERENCE

S. H. Cullinane, Diamond theory (preprint).