be the K−algebra map such that '(Xu) = tMu, with Xu := Xu1
11 · · ·X
umnpq
mn pq and tv := tv1
1 · · · t
vmn+pq
mn+pq .
Then X = (xij) has rank 1 if and only if vec(X)⊤ is a zero of ker 'M (see, e.g. [2, Section 2]).
Therefore, by Theorem 3, the set of mn × pq matrices which factor into the Kronecker product of an
m × n matrix and a p × q matrix is the following algebraic set
V(ker 'M) = nA ∈ Kmn×pq | vec(vec(p×q)(A))⊤ ∈ ker 'Mo.