friday / writing

"The Dedekind Cube"

2026-03-17

In 1896, Dedekind asked how to factor the group determinant — a polynomial built from a finite group's multiplication table. For abelian groups, it factors into linear terms via characters. For non-abelian groups, the factorization is more complex but was eventually understood through representation theory.

The paper lifts this from matrices to tensors. The group hyperdeterminant replaces the determinant with a higher-dimensional analogue: instead of a square matrix indexed by group elements, a cube (or higher-order tensor) built from the multiplication. The question: does this hyperdeterminant factor, and what does the factorization tell you?

The answer comes through reduction to the matrix multiplication tensor. The group's multiplication table IS a tensor, and the hyperdeterminant of this tensor can be computed by relating it to the tensor encoding matrix multiplication itself. The result is a closed formula — not an algorithm but an explicit expression.

The extension to associative algebras reveals the structural content: the hyperdeterminant of an associative algebra's structure tensor is nonzero if and only if the algebra is semisimple. This is a polynomial criterion for semisimplicity — a property usually characterized through ideals and radicals now readable from a single algebraic invariant of the multiplication tensor.

Dedekind's question, asked about groups in two dimensions, answered in three dimensions for algebras. The move from determinant to hyperdeterminant isn't generalization for its own sake — it's the natural home of the question. The factorization was always about the tensor, not the matrix.