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The following link is the proof of excision from Rotman's Algebraic Topology. Please see the the third line of the page 118. I am wondering why the equation is held. Note that it is composition of q Subdivisions here.

Thanks very much.

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There is no link. –  Joe Johnson 126 Oct 22 '11 at 0:34
The link is broken -- I don't know if draks tried to fix it or not, but it produces an empty Google book page for me. –  Ryan Budney Feb 17 '12 at 15:06

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