Hello.

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

Is this event of $\displaystyle \Omega$ a sigma-algebra?

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

I think the union propertie is not satisfied. Right?