Solution
= Solution
For the same Fourier mode, the <leapfrog finite-difference scheme for the diffusion equation> gives
$$
r^{n+1}=r^{n-1}-2qr^n,
\qquad
p(r)=r^2+2qr-1.
$$
The two amplification roots are
$$
r_\pm=-q\pm\sqrt{1+q^2}.
$$
For every nonzero mode $q>0$, one has
$$
|r_-|=q+\sqrt{1+q^2}>1.
$$
Thus every $\mu>0$ admits a growing Fourier mode, and
$$
\boxed{\text{the leapfrog scheme is unstable for every }\mu>0.}
$$