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Could you please tell me what I am supposed to do when facing two sigma ($\Sigma \Sigma$), such as in the covariance formula?

Should I sum the results obtained by each or rather multiply them?

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    Could you give an example so that your question becomes clearer?2012-06-04
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    As stated above an example is the formula for the variance. It looks like this ∑∑... where the first sigma goes from i=1 to n and the second from j=1 to r . Thanks @m-turgeon for answering!2012-06-04
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    What are you summing? In some particular cases, we can simplify.2012-06-04
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    Salut @davide , ce n'est pas un cas particulier qui m'intéresse ici. en effet je voulais juste savoir comment faut-il se comporter face à un double sigma. dois-je sommer ou bien multiplier les résultats obtenus ? O preferivi in italiano? Grazie in anticipo.2012-06-04
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    Non importa (anche se qui la linguia ufficiale è l'inglese, dunque continuo in inglese). If you can write $a_{ij}=b_i\cdot c_j$, then you just multiply the sum over $i$ with the sum over $j$. But is doesn't need to be the case in general. A good way to think about double summations is summation over an array. You can first sum with respect to the rows, then with respect to the columns (and also switch the two sums).2012-06-04
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    thanks indeed @DavideGiraudo2012-06-04

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