Basic Analysis I — Real Analysis · Problem 2.5.5

Sequences and Series — Jiri Lebl

Mathematics — Real Analysis · series-linearity

Problem

For i=1,2,…,ni = 1, 2, \ldots, n, let {xi,k}k=1∞\{x_{i,k}\}_{k=1}^{\infty} denote nn sequences. Suppose that for each i∈Ni \in \mathbb{N}, ∑k=1∞xi,k\sum_{k=1}^{\infty} x_{i,k} is convergent. Prove ∑i=1n(∑k=1∞xi,k)=∑k=1∞(∑i=1nxi,k).\sum_{i=1}^{n} \left( \sum_{k=1}^{\infty} x_{i,k} \right) = \sum_{k=1}^{\infty} \left( \sum_{i=1}^{n} x_{i,k} \right).

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