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I'm currently reading through the eighth edition of A First Course in PROBABILITY, by Sheldon Ross. The section I'm reading is "Momens of the number of events that occur", and I understand everything below except for in the case of the summation of $I_iI_j$, why are they starting from $i<j$?

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If the summation were over all $i\ne j$, then every pair would be counted twice.

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  • $\begingroup$ Are both I's representing events from the same set of events A? $\endgroup$ – Jaigus May 19 at 22:25

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