Multiplication and compact-friendly operators

During the last few years the authors have studied extensively the invarian t subspace problem of positive operators; see [6] for a survey of this inve stigation. In [4] the authors introduced the class of compact-friendly oper ators and proved for them a general theorem on the existence of invariant s ubspaces. It was then asked if every positive operator is compact-friendly In this note, we present an example of a positive operator which is not com pact-friendly but which, nevertheless, has a non-trivial closed invariant s ubspace. In the process of presenting this example, we also characterize the multipl ication operators that commute with non-zero finite-rank operators. We show , among other things, that a multiplication operator M phi commutes with a non-zero finite-rank operator if and only the multiplier function cp is con stant on some non-empty open set.