(and after one more thought about it, one would zero out the row /and column/ to preserve the symmetry of the matrix, if applicable, and thus keep the eigenvalu

(and after one more thought about it, one would zero out the row /and column/ to preserve the symmetry of the matrix, if applicable, and thus keep the eigenvalues real. and one would probably want to think a bit about whether this kind of deletion actually makes sense for the problem... doing real PCA isn't /that/ hard in most cases - I think I would do something this janky only for extremely large matrices or for realtime operation on a microcontroller or something, and then only after thinking hard about it.)