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Apr 7, 2015 at 21:16 comment added Samarkand James, thank you for your answer. I have read the definitions and it seems that phantom maps are $f:X\rightarrow Y$ such that restriction of $f$ to each skeleton is null-homotopic. So, this definition is of interest when $X$ is infinite dimensional. Is there some theory for finite complexes? For example, when $X$ and $Y$ are 2-dimensional, how can we check that map $f:X\rightarrow Y$ satisfy $0=f_*:\pi_2 X \rightarrow \pi_2 Y$.
Feb 13, 2015 at 2:22 history edited James Schwass CC BY-SA 3.0
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Feb 13, 2015 at 1:21 history answered James Schwass CC BY-SA 3.0