4.3.6 - Claimed Equality but Only Proved Subset

[bschneidr]

In this proof, you claim that

$ D_r \circ D_s = \{(a,b) \in R \times R \mid |a-b| < s + r\}$

but you only proved that

$ D_r \circ D_s \subseteq \{(a,b) \in R \times R \mid |a-b| < s + r\}$

To prove your claim, you would need to also prove the following:

$ \{(a,b) \in R \times R \mid |a-b| < s + r\} \subseteq D_r \circ D_s $

I'm not sure whether this can be proved, but I'll submit a pull request if it can be proved or a counterexample can be found.

[psibi]

Thanks for the catch. Would be glad to accept PR. :)