A Short Proof that $M_{n}(A)$ is local if $A$ is local and Fr\'echet
  We give a short and very general proof of the fact that the property of a dense Fr\'echet subalgebra of a Banach algebra being local, or closed under the holomorphic functional calculus in the Banach algebra, is preserved by tensoring with the $n\times n$ matrix algebra of the complex numbers. 
Larry B. Schweitzer