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I do not understand on page 6 of Galois Cohomology from Serre, the comment after exercise 2) part d). He claims that taking G to be the dual of a countably dimensional vector space over $\mathbb{F}_p$ yields an example of a profinite group that is separable but not metrisable.

However this is nothing else than the product of countably many copies of $\mathbb{F}_p$ which obviously satisfies the conditions of part (b) (in fact it should be possible to write down a metric explicitly).

I am guessing he meant to hint that an uncountable power of $\mathbb{F}_p$ gives such an example (e.g. one views the G he writes as discrete group, takes once more the algebraic dual and then gets the group he claims), which indeed works.

Am I misunderstanding something? Is there an errata of this book somewhere?

Rodolphe
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  • Serre writes "bidual", not "dual". – YCor Jan 17 '24 at 15:14
  • Most definitely not in the version I have! – Rodolphe Jan 17 '24 at 15:16
  • I have an edition "Corrected second printing 2002 of the first English edition of 1997". – YCor Jan 17 '24 at 15:17
  • Oh, I see. Mine seems to be from 1996 (judging from Serre's foreword). Glad that this got corrected in a later edition. OK, then I'll try to get a later edition as perhaps there other misprints/typos in the one I have. Thanks a lot! – Rodolphe Jan 17 '24 at 15:22
  • Normally you have in the very first pages an "official" edition year. The 1996 foreword is also copied (including the date) in my edition, so it says nothing on which edition you have now. Check the previous page. – YCor Jan 17 '24 at 15:23
  • Uhm, checked it up and seems to be indeed 1997, which is very strange. I am staring right now at the file in front of me and I am reading "dual" not "bidual". – Rodolphe Jan 17 '24 at 15:29
  • Oh sorry now I read your comment about 2002! No I don't have that. So that explains! – Rodolphe Jan 17 '24 at 15:30
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    In any case "bidual" is a bit vague (he indeed takes the dual, endows it with the discrete topology, and then endows the bidual with the compact topology). It is more clear to describe it as an uncountable product of at most continuum copies of $\mathbf{F}_p$. – YCor Jan 17 '24 at 15:33
  • Yes, I very much agree with you! – Rodolphe Jan 17 '24 at 15:35

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