= Einstein notation
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= Einstein summation convention
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The https://en.wikipedia.org/w/index.php?title=Einstein_notation&oldid=1021244532[Wikipedia page] of this article is basically a masterclass why <ourbigbook com/Wikipedia>[Wikipedia is useless for learning technical subjects]. They are not even able to teach such a simple subject properly there!
Bibliography:
* https://www.maths.cam.ac.uk/postgrad/part-iii/files/misc/index-notation.pdf gives a definition that does not consider upper and lower indexes, it only counts how many times the indices appear
Their definition of the <Laplacian> is a bit wrong as only one $i$ appears in it, they likely meant to have written $\pdv{}{x_i}\pdv{F}{x_i}$ instead of $\pdv{^2 F}{x_i^2}$, related:
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