= Velocity ellipsoid
{title2=$\sigma_{ij}^2=\langle(v_i-\bar v_i)(v_j-\bar v_j)\rangle$}
The <velocity ellipsoid> represents the positive <velocity> <covariance> tensor by its orthogonal principal axes and eigenvalue square roots. The uncentered second-moment tensor differs from <covariance> when a population streams. Vanishing mixed <second moments> align the second-moment <ellipsoid> with the chosen frame; for pure azimuthal streaming in a phase-mixed axisymmetric model, the off-diagonal <covariance> also vanishes.
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