  The characteristics of the spectral evolution of the prompt emission of
gamma-ray bursts (GRBs), which are closely related to the radiation mechanism
(synchrotron or photosphere), are still an unsolved subject. Here, by
performing the detailed time-resolved spectral fitting of GRB 131231A, which
has a very bright and well-defined single pulse, some interesting spectral
evolution features have been found. (i) Both the low-energy spectral index
$\alpha$ and the peak energy $E_{\rm p}$ exhibit the "flux-tracking" pattern
("double-tracking" characteristics). (ii) The parameter relations, i.e., $F$
(the energy flux)-$\alpha$, $F$-$E_{\rm p}$, and $E_{\rm p}$-$\alpha$, along
with the analogous Yonetoku $E_{\rm p}$-$L_{\gamma,\rm iso}$ relation for the
different time-resolved spectra, show strong monotonous (positive)
correlations, both in the rising and the decaying phases. (iii) The values of
$\alpha$ do not exceed the synchrotron limit ($\alpha$= -2/3) in all slices
across the pulse, favoring the synchrotron origin. We argue that the one-zone
synchrotron emission model with the emitter streaming away at a large distance
from the central engine can explain all of these special spectral evolution
characteristics.
