# Question about liminf of the sum of two sequences

1. Oct 22, 2009

### AxiomOfChoice

I know that for any two real sequences x_n and y_n, we have

$$\liminf_{n\to \infty} x_n + \liminf_{n\to \infty} y_n \leq \liminf_{n\to \infty} (x_n + y_n).$$

I also know that, if one of the sequences converges, the inequality becomes equality. My question is this: If I've managed to show that

$$\liminf_{n\to \infty} x_n + \liminf_{n\to \infty} y_n = \liminf_{n\to \infty}(x_n + y_n),$$

can I conclude that one, or both, of the sequences converge? A simple yes/no would suffice, but (of course) I'd prefer a short proof or counterexample. Thanks!

2. Oct 22, 2009

What if

$$x_n = y_n = (-1)^n$$

3. Oct 22, 2009

### AxiomOfChoice

Lame! I was hoping both of the sequences had to converge! And what a simple counterexample to prove me wrong! Thanks, though