= Solution
For the Fourier mode $e^{i(i\theta+j\phi)}$, the <Forward Euler method> has <amplification factor>
$$
H(\theta,\phi)=1-4\mu\left[\sin^2(\theta/2)+c\sin^2(\phi/2)\right].
$$
It is real and ranges from $1-4\mu(1+c)$ to $1$. The two-dimensional <Fourier stability analysis> therefore requires its minimum to be at least $-1$. Thus \b[the exact stable range is]
$$
\boxed{0<\mu\leq\frac1{2(1+c)}.}
$$
The endpoint is included; the highest-frequency mode then has amplification $-1$ and bounded norm.
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