Alon Shmiel
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 Jan 18 awarded Promoter Nov 28 accepted Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. Nov 28 comment Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. I know why but i dont know how to explain it.. the answer is: A doesn't belong to F because of the definition of F.. Nov 28 revised Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. edited title Nov 28 comment Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. can you give me an exaple of A and F? I think I dont really know what is F.. thank you! Nov 28 comment Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. @DonAntonio, you are right. I translated the word to english and use a group instead of a set. Nov 28 revised Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. deleted 4 characters in body Nov 28 asked Given a set $A$ such that for any family of sets $F$: if $\cup F = A$ then $A \in F$. Nov 25 comment Prove that if $|a_n-a_{n-1}| < \frac{1}{2^{n+1} }$ and $a_0=\frac12$, then $\{a_n\}$ converges to \$0