Which of the following metric spaces are complete?

metric-spaces complete-spaces cauchy-sequences

For (1), consider the sequence $\left\langle\frac1{2^n}:n\in\Bbb N\right\rangle$. Is it $d$-Cauchy? Does it converge to anything in $X_1$?

For (4), what about $\langle -n:n\in\Bbb N\rangle$?

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