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Let $X$ be a subcomplex of a simplicial complex $Y$ such that $X\simeq \mathbb{S}^6$ and $Y\simeq \mathbb{S}^7\vee\mathbb{S}^4$.

Question: Is the inclusion map $i :X \longrightarrow Y$ always null homotopic?

Any ideas or references are greatly appreciated!

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    $\begingroup$ No. $\pi_6(S^7\vee S^4)\cong \pi_6(S^4)\cong \mathbb Z/2$. There is a non-trivial map $S^6\to S^7\vee S^4$. Using the mapping cylinder construction and simplicial approximation you can realize this map as an inclusion of a simplicial subcomplex. $\endgroup$ Commented Mar 25, 2021 at 8:31
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    $\begingroup$ Maybe you should post this as an answer (I don't think there's anything to be added at all) $\endgroup$ Commented Mar 25, 2021 at 15:08
  • $\begingroup$ Thank you @GregoryArone! $\endgroup$ Commented Mar 26, 2021 at 8:00

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