||x_n - x_m|| >= ||x_n_k - x_m_k||
So, if (x_n) is a Cauchy sequence, then so are (x_n_k) for all k.
Since R is complete every (x_n_k) converges against a number y_k.
Now show that (x_n) converges against y:=(y_1,y_2,...) in the p-norm.
The book gave me the "hint" to mimic the proof of the l(1) space, but I can't find it anywhere. I have no idea how to approach this. I understand I want to show that every cauchy space converges to something in l(p), or that every sequence of sequences becomes arbitrarily close to another sequence. I know it follows from completeness on R, but thats all I know. I'm totally lost.