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  "body": "\n## Summary\n\nA relative CW complex is a pair $(X, A)$ of spaces in which $X$ is built from $A$ by attaching cells of increasing dimension.\nWhen $A$ is empty, it is the same as a [[CW complex|5W95QFHXPR]].\nThe inclusion $A \\to X$ of a relative CW complex has the homotopy extension property, so it is a cofibration.\n\n## Explanations\n\n### L1 human\n\nStart with any space $A$, not necessarily a CW complex.\nGlue points to it, then intervals along their ends, then disks along their boundary circles, and so on.\nThe result $X$ is a relative CW complex over $A$.\nFor example, the closed disk $D^2$ is obtained from its boundary circle $S^1$ by gluing one $2$-cell, so $(D^2, S^1)$ is a relative CW complex.\n",
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