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I have stumbled upon some interesting exercise whilst reading the "Category Theory for Scientists" book.

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Below is the universal property of fiber products:

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By using the universal property, I can try placing some set A, connect it to the X, X', Y, Y' through some functions and thus have two unique arrows spawning from it to the W and W'. However, nothing that could be used to create a direct connection between the W and W' comes out from that.

Any suggestions how this could be solved?


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2 Answers 2

up vote 3 down vote accepted

Consider the morphisms $W \to Y \to Y'$ and $W \to X \to X'$. Evidently they induce the same morphism $W \to Z'$. Hence, the universal property of $W'$ tells us that that they induce a morphism $W \to W'$ such that everything commutes (we don't need even need that $W$ is a pullback).

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So basically the "W" pullback in the exercise can be looked at as the "A" set in the universal property definition, and the W -> Y -> Y' and W -> X -> X' morphisms as the "f" and "g" functions? :) –  spacemonkey Feb 7 at 0:11
Yes. Remember that names of variables can be freely interchanged. –  Martin Brandenburg Feb 7 at 0:46
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Hint: Since the square on the face behind is a pullback, look for a pair of maps $W\to Y',\ W\to X'$ to form a commuting square with $Y'\to Z'$ and $X'\to Z'$.

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