文摘
The assignment (nonstable K0-theory), that to a ring聽R associates the monoid聽V(鈥?em class="a-plus-plus">R鈥? of Murray-von Neumann equivalence classes of idempotent infinite matrices with only finitely nonzero entries over聽R, extends naturally to a functor. We prove the following lifting properties of that functor: There is no functor 螕, from simplicial monoids with order-unit with normalized positive homomorphisms to exchange rings, such that V 鈭樷€壩撯€夆墔 id. There is no functor 螕, from simplicial monoids with order-unit with normalized positive embeddings to C*-algebras of real rank聽0 (resp., von Neumann regular rings), such that V 鈭樷€壩撯€夆墔 id. There is a {0,1}3-indexed commutative diagram聽 ${\vec{D}}$ of simplicial monoids that can be lifted, with respect to the functor聽V, by exchange rings and by C*-algebras of real rank聽1, but not by semiprimitive exchange rings, thus neither by regular rings nor by C*-algebras of real rank聽0. By using categorical tools (larders, lifters, CLL) from a recent book from the author with P. Gillibert, we deduce that there exists a unital exchange ring of cardinality聽 $\aleph_3$ (resp., an $\aleph_3$ -separable unital C*-algebra of real rank聽1)聽R, with stable rank聽1 and index of nilpotence聽2, such that聽V(鈥?em class="a-plus-plus">R鈥? is the positive cone of a dimension group but it is not isomorphic to聽V(鈥?em class="a-plus-plus">B鈥? for any ring聽B which is either a C*-algebra of real rank聽0 or a regular ring.