Help understanding set exponential notation

I'm reading through some notes which give a proof by induction for Bool's inequality, and I wondered if someone could explain what the notation. I'm confused by the E^(k), what does this mean?

Since I'm a new user, here is a link to the image. http://i.stack.imgur.com/L0mCj.png

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In the context of the formula you linked to, it means $\displaystyle\bigcup_{i=1}^k E_k$.
The brackets are to distinguish it from $E^k$, which has a pretty standard meaning of direct product $E\times E\times\cdots\times E$, as in $\mathbb{R}^3$. –  André Nicolas Dec 29 '12 at 16:29