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Geometric interpretation for this invariant in square bracket notation?
Let $X=\{p_1,p_2,p_3,p_4\}$ be four points in $\mathbb{R}^2$, not three of them on a line. We use the square bracket notation
$$[i,j,k]=\det \begin{pmatrix} p_i & p_j & p_k \\ 1 & 1 & ...
