mm-1919 === operator and surjective T maps open sets to open sets. What about if you drop the hypothesis that T is surjective, is T(0), O open in X, Borel? Jos.8e Gasc.97n Originator: bergv@math.uiuc.edu (Maarten Bergvelt) No. from one Banach space to another such that the image of the closed unit ball is not a Borel set. It then follows that the image of the open ball is not Borel, since any closed ball is a countable intersection of open balls. Taking example mapping a Banach space to itself. Let Y be l^2, say, nicely rotund. Let S be the unit sphere in Y, S = { y : ||y|| = 1 }, a closed set. Let Q be a nasty subset of S, non-Borel. Let X = l^1(Q), the space of functions absolutely. T is linear and ||T|| = 1. Now I claim the closed ball B = { f in X : ||f|| <= 1 } maps onto a non-Borel set in Y. Indeed, the intersection of T(B) with the closed set S is exactly Q, so T(B) is not Borel. -- G. A. Edgar http://www.math.ohio-state.edu/~edgar/ Originator: bergv@math.uiuc.edu (Maarten Bergvelt) ì.94 .b2«¨¡.b3¨.b2[Degree ]¡[Micro] ¡.b3.bc.b3¡Â æ.95.97.8c .82.87.97.8b.95.94 Ö.98.95.99.8c.bc operator and surjective T maps open sets to open sets. What about if you drop the hypothesis that T is surjective, is T(0), O open in X, Borel? Jos,bi(B Gasc,bs(Bn think your assertion is right, if in addition you assume X to be .97.8c[Eth].87.98.87.89.93.8c[ RegisteredTrademark] hope that I remember the following theorem correctly: An injective å.95.98.8csubsets of A to such of B. This should be found in Ëåìì[Capi talEDoubleDot].9e[CapitalEth].87.98.99.8f.87 .97.87.98.87.99.8f.9d±.b9 .a6á .99.8e.99.93= {Probability measures on metric spaces}, [Eth].9b.89.93.8e.97.8f.8c= {Academic Press}, .9d.8c.87.98 {1967}, .87.9b.99.8f.95.98 {Kalyanapuram R. Parthasarathy}, [YAcute]æ X-ASG-Orig-Subj: Re: skew fields Originator: bergv@math.uiuc.edu (Maarten Bergvelt) all This is too complicated: the multiplicative group of the field R (real numbers) is not residually finite. Thus you do not need non-commutativity. Here is a proof that R^* is not residually finite. Suppose that there exists a homomorphism f from R^* to a finite group separating 2 from 1. Suppose that a root a of 2 of degree k^m and a root b of 2 of degree k^n have the same images under f. Thus f(a)=f(b). Raising to the power k^m gives f(2)=f(b^(k^m))=f(2^(k^(m-n)))=1, a contradiction. Perhaps a more appropriate question is: are there skew fields K for which not every finitely generated subgroup of K^* is residually finite? Mark Originator: bergv@math.uiuc.edu (Maarten Bergvelt) Kadec-Klee property is the more modern term. For a history of the development of the concept, starting with Radon, see the PDF file at