Symmetry transformations for square sliced three-way arrays, with applications to their typical rank

The typical 3-tensorial rank has been much studied over algebraically closed fields, but very little has been achieved in the way of results pertaining to the real field. The present paper examines the typical 3-tensorial rank over the real field, when the slices of the array involved are square matrices. The typical rank of 3 × 3 × 3 arrays is shown to be five. The typical rank of p × q × q arrays is shown to be larger than q + 1 unless there are only two slices (p = 2), or there are three slices of order 2 × 2 (p = 3 and q = 2). The key result is that when the rank is q + 1, there usually exists a rank-preserving transformation of the array to one with symmetric slices.