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This is a notational question:

What does $\overset{\underset{\mathrm{\Delta }}{}}{=}$ denote in

$-0.5JDJ\overset{\underset{\mathrm{\Delta }}{}}{=} X^TX$ where $J=I-n^{-1}ee^T$ with $e$ being a vector of $1's$ and $D$ is a matrix of squared distances of the rows in $X$ i.e, $D_{ij}=||X_{i.}-X_{j.}||^2$

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up vote 1 down vote accepted

The symbol usually means 'is defined as' or 'is another name for'. Do you see why that might make sense in your context?

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