Derivation of an equation of Influential Analysis in OLS

Hi to everyone, I'm new to this forum and I'm very pleased for the existence of this forum, since the fact that forums in these times are in decay. I'm not a native English speaker so I ask for a little of patience lol

I'm studying Ordinary Least Squares, and in the section of Influential Analysis of the class notes, we have an expression to detect influential observations (more details in the image)

EDIT: Corrected image

I do not have any idea of the origin of these expressions and obviously their derivations (the class notes have a lot of references, but mostly they are difficult to access). They have a name? Are they a specific theorem?

Thanks a lot: wave:
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Can't make spagetti
Cleared from moderator's queue!

BTW, your link doesn't show any pictures for me. I mean, it opens the page but it doesn't display anything.
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