  Sonine kernel is characterized by the Sonine condition (denoted by SC) and is
an important class of kernels in nonlocal differential equations and integral
equations. This work proposes a SC with a more general form (denoted by gSC),
which is more convenient than SC to accommodate complex kernels and equations.
A typical kernel is given, and the first-kind Volterra integral equation under
gSC is accordingly transformed and then analyzed. Based on these results, it is
finally proved that the gSC is indeed equivalent to the original SC, which
indicates that the Sonine kernel may be essentially characterized by the
behavior of its convolution with the associated kernel at the starting point.
