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Here is my question, which is part of the beginning of the chi-squared decomposition theorem proof : Suppose that $P_1, ....P_k $ are symmetric projection matices with sum the identity:

$I = P_1 + P_2 + ... + P_k $

Then squaring,

$ I = I^2 = \sum_i P_i^2 + \sum_{i

My question is why after squaring, it yields the term $\sum_{i

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    Should it be $2 \sum_{i < j} P_{i} P_{j}$?2012-11-25
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    Yes, I think so, but my lecture notes dont have the coefficient 2, which puzzled me da whole night ==.2012-11-25

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Actually, under your conditions, $$I=I^2=\sum_{_i}P_i + 2\sum_{i

This is simple algebra $(P_1+...+P_n)^2=(P_1+...+P_n)(P_1+...+P_n)$.

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    Ok. Because in my lecture notes, the coefficient 2 is missing. So , it must be wrong then. Thanks2012-11-25
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    I don't see why any two symmetric matrices should commute.2014-08-05