# General formula for calculating $\prod_{i \in I}(1-a_i)$

Does anyone know the general formula for calculating $\prod_{i \in I}(1-a_i)$ where $I$ is a set of indices?
I suspect it is $1+\sum_{\emptyset \subsetneq J \subseteq I}(-1)^{|J|}\prod_{j \in J}a_j$. I don't require a proof, just a confirmation.

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Looks almost fine. You should write either $\sum_{\emptyset \ne J \subseteq I}$ or omit the "$1$", as $(-1)^{|\emptyset|}\prod_{j \in \emptyset} a_j = 1$ gives the second "$1$" now, which you don't need. –  martini Sep 28 '12 at 7:20
Done the edit as suggested. –  Abhishek Anand Sep 28 '12 at 7:23
Now you are right. –  martini Sep 28 '12 at 7:45