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I have from my class notes,

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But when I search the web I can't find other sources that give this same formula for correlation. The wikipedia article on correlation and dependence just shows the correlation coefficient. Would someone please explain the difference to me and why I can't find the $E[XY]$ definition of correlation on wikipedia?

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    I don't think anyone would define "correlation" between $X$ and $Y$ as $\operatorname{E}(XY)$ EXCEPT in a context where it's already been made clear that $X$ and $Y$ both have expected value $0$ and variance $1.\qquad$ – 2017-01-29

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