On ultrapowers of Banach spaces of type L∞

We prove that no ultraproduct of Banach spaces via a countably incomplete ultrafilter can contain c(0) complemented. This shows that a "result" widely used in the theory of ultraproducts is wrong. We then amend a number of results whose proofs have been infected by that statement. In particular we provide proofs for the following statements: (i) All M-spaces, in particular all C(K)-spaces, have ultrapowers isomorphic to ultrapowers of c(0), as also do all their complemented subspaces isomorphic to their square. (ii) No ultrapower of the Gurarii space can be complemented in any M-space. (iii) There exist Banach spaces not complemented in any C(K)-space having ultrapowers isomorphic to a C(K)-space.