Searching for a preferred direction with Union2.1 data
  A cosmological preferred direction was reported from the type Ia supernovae
(SNe Ia) data in recent years. We use the Union2.1 data to give a simple
classification of such studies for the first time. Because the maximum
anisotropic direction is independent of isotropic dark energy models, we adopt
two cosmological models ($\Lambda$CDM, $w$CDM) for the hemisphere comparison
analysis and $\Lambda$CDM model for dipole fit approach. In hemisphere
comparison method, the matter density and the equation of state of dark energy
are adopted as the diagnostic qualities in the $\Lambda$CDM model and $w$CDM
model, respectively. In dipole fit approach, we fit the fluctuation of distance
modulus. We find that there is a null signal for the hemisphere comparison
method, while a preferred direction ($b=-14.3^\circ \pm 10.1^\circ,
l=307.1^\circ \pm 16.2^\circ$) for the dipole fit method. This result indicates
that the dipole fit is more sensitive than the hemisphere comparison method.
