In the following, "X", "Y", "Z", "W" are Banach spaces, B("X", "Y") is the space of bounded operators from "X" to "Y" with the operator norm, K("X", "Y") is the space of compact operators from "X" to "Y", B("X") = B("X", "X"), K("X") = K("X", "X"), formula_5 is the identity operator on "X".