Denote by ‖ ‖ x ‖ ‖ 0 the number of non-zero elements in the time sequence x 0 , x 1 , … … , x N − − 1 and by ‖ ‖ X ‖ ‖ 0 the number of non-zero elements in the frequency sequence X 0 , X 1 , … … , X N − − 1 . Then, ‖ ‖ x ‖ ‖ 0 ⋅ ⋅ ‖ ‖ X ‖ ‖ 0 ≥ ≥ N .