# Summable Sequences

1. Homework Statement
Determine whether or not the sequences below are summable:

{(-1)^n}

{(-1)^n + (-1)^(n+1)}

{(-1)^n} + {(-1)^(n+1)}

2. Homework Equations

3. The Attempt at a Solution

Okay, I'm having some trouble thinking about these the right way. Since

{(-1)^n}= -1, 1, -1, 1, .... then its sum = -1, 0, -1, 0, -1.

I think this means that it is not summable even though it is 0 every other term.

Assuming it is divergent, then {(-1)^(n+1)} is of course also divergent... But I think that two divergent sequences added together might be convergent.

But does it matter whether they are summed together as one sequence or two? Either way they will still = 0, 0, 0, 0, 0 .... right? So would they both be summable? Sorry if I sound confused - it's just because I am.

## Answers and Replies

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tiny-tim
Homework Helper
… you can't add things if they don't exist …

But does it matter whether they are summed together as one sequence or two? Either way they will still = 0, 0, 0, 0, 0 .... right? So would they both be summable? Sorry if I sound confused - it's just because I am.
Hi alovesong!

(it's ok so long as you know you're confused! )

Yes it does matter.

… you can't add things if they don't exist …

∑{An} + ∑{Bn} is only defined if both ∑{An} and ∑{Bn} are defined.

Even though ∑{An + Bn} is defined!