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Let $A,B$ be groups.Can you explain why $U\le A \times B$ does not imply $U=\left(A\cap U\right) \times \left( B \cap U \right)$ this is an exercise in the book of the theory of finite groups an introuduction written by H.Kurzweil. the meaning of each symbol may as follows. $A\times B$ is the direct product of $A$,$B$. $U$ is the subgroup of $A\times B$ , thus $U= \lbrace\left(a,b\right)|a \in A,b\in B \rbrace$, $A \cap U=\lbrace a_1|\left(a_1,b_1\right) \in U,a_1\in A \rbrace$,in the same way we could know $\left( B \cap U \right)$

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    @JonasMeyer ZFC is crappy foundation in this regard.2012-12-28

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Think about it intuitively.

Take $A,B = \mathbb{R}$. The LHS $U$ is "some set of points in the plane" whereas the RHS is "all possible x-coordinates from points in $U$ with all possible y-coordinates from points in $U$".

Clearly $U\subseteq$ RHS but the RHS can be bigger.

For a specific example let $U = \{(0,1),(1,0)\}$. Then the RHS is $\{(0,0),(0,1),(1,0),(1,1)\}$.

The point is that the object on the RHS is combining things componentwise that might not have existed when we chose "special" points to be in $U$.

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    Oh I didn't know that, I alwa$y$s thought it was just convention which symbol to use when writing maths on a computer. $A$nyway, the property is highly unlikely to hold for sets so it is easy to tweak and find groups for which it doesn't hold.2012-12-30
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How about a simple example such as $A=B\ne1$ and $U=\{(x,x)\mid x\in A\}$?

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    Stupid me, I think I might have had an off day with this question.2012-12-29