# Questions tagged [projection]

For on-topic questions involving the mathematical concept projection, a linear transformation $P$ such that $P=P^2$. Please include also a more statistical methods tag. For purely mathematical questions about projections it is better to ask on math SE https://math.stackexchange.com/

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### Cross-covariance in context of Andrews plot

As shown in this Cross-Validated post Close curves on an Andrews plot I don't understand how, in the accepted answer, the cross-covariance can be defined as, $$\int_{-\pi}^{\pi}f_xf_ydt$$ Considering ...
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### Range of error in linear regression [closed]

In linear regression whose matrix form is $$Y = Xb + E$$ where $X=[X_1,X_2]$. If the error of using only $X_1$ (i.e $Y = X_1b+E$ ) is $x_1$, and the error of using $X_2$ (i.e $Y=X_2b+E$) is $e_2$, ...
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### What is the relationship between the conditional expected value, OLS and projection in linear regression?

I am learning about the linear regression. I was taught it from two perspectives. The first one was about an equation connecting the conditional expected value and the predictor. I saw the nice graphs ...
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### Projection operator: why squared norm of the sum of them is equal (or smaller) than the sum of the squared norms?

I am working through the proof of Lemma 2 in this paper (page 25, need it for my own research) and I am stuck at the very first step. Here, I will formulate a bit simplified version of this step. ...
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