feature engineering for dark-matter halos

Kate Storey-Fisher and I spoke about adding eigenvalues (scalars) and eigenvectors (vectors) to our geometric features of dark-matter halos. For regression! But the problem with this is that the eigenvectors have ambiguous sign; is that an issue? Yes! They are descriptions of an order-2 tensor, not order-1 vectors. Hmmm. We also spoke about whether the feature engineering should be deterministic, or chosen by hand.