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I'm trying to teach myself QFT, and I'm stuck with one formula in Part

I of Weinberg's trilogy.

I think I managed to understand how one gets Formula 2.B.7 in the

appendix of Chapter 2, thanks to the information provided by Weinberg,

but don't get 2.B.10, page 97. Could anyone one give me a hint on how

to get it? FWIW, I'm using the paperback edition.

Since this is not key issue, but mostly a technical mathematical point,

I wouldn't want to get bogged down too long by this, but this is

getting irritating :-)

TIA,

Pierre

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# Homotopy classes appendix in Weinberg's QFT book, Part I

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