No evidence for dynamical dark energy in two models
  To investigate whether the dark energy evolves over time, we propose two null
tests and constrain them using the data combination of cosmic microwave
background radiation, baryonic acoustic oscillations, Type Ia supernovae,
Planck-2015 lensing and cosmic chronometers. We find that, for these two
models, there is no evidence of the dynamical dark energy at the $1.2\sigma$
confidence level. Interestingly, both models could slightly alleviate (i) the
current Hubble constant ($H_0$) tension between the global fitting derivation
by the Planck collaboration and the local observation by Riess {\it et al.};
(ii) the root-mean-square density fluctuations ($\sigma_8$) tension between the
Planck-2015 data and several low-redshift large scale structure probes.
