Theorem 3. Let A = (Aij) be a nonzero mp × nq matrix, partitioned into blocks of order p × q.
There exist matrices B (of order m × n) and C (of order p × q) such that A = B ⊗ C if and only if
rank(vec(p×q)(A)) = 1.
Proof. If A = B ⊗ C is a factorization of the stated form, then B,C, vec(B), and vec(C) must all be
nonzero. Lemma 2 ensures that
rank(vec(p×q)(A)) = rank(vec(p×q)(B ⊗ C)) = rank(vec(B)vec(C)⊤) = 1.
Conversely, since A 6= 0, there are indices r and s such that Ars 6= 0 and hence vec(Ars) 6= 0. Since
rank(vec(p×q)(A)) = 1, each row of vec(p×q)(A) is a scalar multiple of any nonzero row. Thus, there
are scalars bij such that each vec(Aij ) = bij vec(Ars). This means that A = B ⊗C, in which B = (bij)
and C = Ars.
Notice that the preceding proof provides a simple construction for a pair of Kronecker factors for
A if rank(vec(p×q)(A)) = 1
Theorem 3. Let A = (Aij) be a nonzero mp × nq matrix, partitioned into blocks of order p × q.There exist matrices B (of order m × n) and C (of order p × q) such that A = B ⊗ C if and only ifrank(vec(p×q)(A)) = 1.Proof. If A = B ⊗ C is a factorization of the stated form, then B,C, vec(B), and vec(C) must all benonzero. Lemma 2 ensures thatrank(vec(p×q)(A)) = rank(vec(p×q)(B ⊗ C)) = rank(vec(B)vec(C)⊤) = 1.Conversely, since A 6= 0, there are indices r and s such that Ars 6= 0 and hence vec(Ars) 6= 0. Sincerank(vec(p×q)(A)) = 1, each row of vec(p×q)(A) is a scalar multiple of any nonzero row. Thus, thereare scalars bij such that each vec(Aij ) = bij vec(Ars). This means that A = B ⊗C, in which B = (bij)and C = Ars. Notice that the preceding proof provides a simple construction for a pair of Kronecker factors forA if rank(vec(p×q)(A)) = 1
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