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"Let ${x_n}subset U$ be a Cauchy sequence. Give a direct proof that if a subsequence ${x_{nk}}subset S$ has a limit $L$, then the Cauchy sequence ${x_n}subset U$ has $L$ as a limit. Do not assume that general Cauchy sequences are convergent."

Ok I know a lot of people hate helping others with homework when they feel that the person asking the question hasn't worked on the problem. I have spent about 30 minutes going through all my theorems from my textbook on Cauchy sequences and I don't know where to start. Please help me

Let $x_n$ be a Cauchy sequence and $(x_n)_k$ a subsequence. Let $epsilon >0$. Then for sufficiently large $n$, since $x_n$ is Cauchy, $|x_n-(x_n)_k|<epsilon /2.$ Since the subsequence converges to $L$ we have $|(x_n)_k-L|<epsilon /2$ for sufficiently large $k$. Choosing the maximum of $n$ for which the above are satisfied gives $|x_n-L|=|x_n-(x_n)_k+(x_n)_k-L|leq |x_n-(x_n)_k|+|(x_n)_k-L|<epsilon$.

The actual theorem states that a Cauchy sequence is convergent iff it has a convergent subsequence, and your question only asks us to prove the if part. It follows rather immediately from the triangle inequality. Pick $n_k$ such that $|x_{n_k}-L|leqdfrac{epsilon}{2}$, and $n,m$ such that $|x_n-x_m|leq dfrac{epsilon}{2}$. You should be able to see why these exist for integers sufficiently large. Then

$$|x_n-L|leq|x_n-x_m|+|x_{n_k}-L|$$.

Fix $epsilon>0.$ $x_{n_k}rightarrow L$ implies there is an integer $k_0$ such that $d(x_{n_k},L)<epsilon/2$ for $kgeq k_0.$ $(x_n)$ is Cauchy implies there is an integer $n_0$ such that $d(x_m,x_n)<epsilon/2$ for $m,ngeq n_0.$ Take $N=max{n_0,n_{k_0}}.$ Then for $ngeq N,$ $d(x_n,L)leq d(x_n,x_{n_{k_0}})+d(x_{n_{k_0}},L)<epsilon.$ Hence $x_nrightarrow L.$