In their recent paper “The spectral norm of a Horadam circulant matrix”, Merikoski, Haukkanen, Mattila and Tossavainen study under which conditions the spectral norm of a general real circulant matrix C equals the modulus of its row/column sum. We improve on their sufficient condition until we have a necessary one. Our results connect the above problem to positivity of sufficiently high powers of the matrix C>C. We then generalize the result to complex circulant matrices.
