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Sigma Notation
Sigma means “to sum up”. It’s a way of writing long summations in a concise way. It works very similar to a “for” loop in programming, it’s just that each “loop iteration” adds itself to the previous.
$$ \displaystyle\sum_{n=1}^4 n^2=? $$This is the same as:
$$ 1^2+2^2+3^2+4^2 $$Be careful not to confuse this with Sigmoid function - definitely not the same thing.