  We revisit the phenomenon of instability of solitons in the two dimensional
generalization of the Korteweg-de Vries equation, the generalized
Zakharov-Kuznetsov (ZK) equation, $u_t + \partial_{x_1} (\Delta u + u^p) = 0,
(x_1,x_2) \in \mathbb R^2$. It is known that solitons are unstable in this two
dimensional equation for nonlinearities $p > 3$. This was shown by Anne de
Bouard in [4] generalizing the arguments of Bona-Souganidis-Strauss in [1] for
the generalized KdV equation. In this paper, we use a different method to
obtain the instability of solitons, namely, truncation and monotonicity
properties. Not only does this approach simplify the proof, but it can also be
useful for studying various other stability questions in the ZK equation as
well as other generalizations of the KdV equation.
