Perturbation of Angles Between Linear Subspaces

We consider in this note how the principal angles between column spaces R(A) and R(B) change when the elements in A and B are subject to perturbations. The basic idea in the proof of our results is that the non-zero cosine values of the principal angles between R(A) and R(B) coincide with the non-zero singular values of $P_AP_B$, the product of two orthogonal projections, and consequently we can apply a perturbation theorem of orthogonal projections proved by the author4].