  We show that there are at most two solutions in positive integers $(x,y,z)$
to the equation $a^x+b^y=c^z$ for positive integers $a$, $b$, and $c$ all
greater than one, where at least one of $a$, $b$, $c$ is not a power of 2, and
$(a,b,c) \ne (3,5,2)$. The case in which $\gcd(a,b)=1$ has been handled by in a
series of successive results by Scott and Styer, Hu and Le, and Miyazaki and
Pink, who showed that there are at most two solutions except for $(a,b,c) =
(3,5,2)$, which has three solutions. So here we treat the case $\gcd(a,b)>1$,
showing that there are at most two solutions in this case if at least one of
$a$, $b$, $c$ is not a power of 2 (two solutions $(x_1,y_1,z_1)$ and
$(x_2,y_2,z_2)$ are considered the same solution if $a^{x_1} = b^{y_2}$ and
$a^{x_2} = b^{y_1}$). This generalizes work of Bennett who proved the
equivalent result (for both $\gcd(a,b)=1$ and $\gcd(a,b)>1$) for the case in
which either $x=1$ or $y=1$.
