Procrustes problems and associated approximation problems for matrices with k-involutory symmetries

We say that a matrix R∈Cn×n is k-involutary if its minimal polynomial is xk-1 for some k⩾2, so Rk-1=R-1 and the eigenvalues of R are 1,ζ,ζ2,…,ζk-1, where ζ=e2πi/k. Let α,μ∈{0,1,…,k-1}. If R∈Cm×m, A∈Cm×n, S∈Cn×n and R and S are k-involutory, we say that A is (R,S,μ)-symmetric if RAS-1=ζμA, and A is (R,S,α,μ)-symmetric if RAS-α=ζμA.Let L be the class of m×n (R,S,μ)-symmetric matrices or the class of m×n (R,S,α,μ)-symmetric matrices. Given X∈Cn×t and B∈Cm×t, we characterize the matrices A in L that minimize ‖AX-B‖ (Frobenius norm), and, given an arbitrary W∈Cm×n, we find the unique matrix A∈L that minimizes both ‖AX-B‖ and ‖A-W‖. We also obtain necessary and sufficient conditions for existence of A∈L such that AX=B, and, assuming that the conditions are satisfied, characterize the set of all such A.