# Math Help - Sigma-algebra exercise.

1. ## Sigma-algebra exercise.

Hello

We have the following sample space: $\Omega = \left\{{1,2,3}\right\}$

Is the following event of $\Omega$ a sigma-algebra?

$
A = \left\{{\left\{{\emptyset}\right\},\left\{{1,2,3}\ right\},\left\{{1}\right\},\left\{{2}\right\},\lef t\{{1,3}\right\},\left\{{2,3}\right\}}\right\}
$

I think the union property is not satisfied. Right?

Thanks

2. Please do not post twice. EDIT the OP.