Consider the problem
$$
\begin{aligned} \alpha^{2} u_{x x}=u_{t}, & 00 \\ u(0, t)=0,
\quad u_{x}(L, t)+\gamma u(L, t)=0, & t>0 \\ u(x, 0)=f(x), & 0 \leq x \leq L
\end{aligned}
$$
(a) Let \(u(x, t)=X(x) T(t)\) and show that
$$
X^{\prime \prime}+\lambda X=0, \quad X(0)=0, \quad X^{\prime}(L)+\gamma X(L)=0
$$
and
$$
T^{\prime}+\lambda \alpha^{2} T=0
$$
where \(\lambda\) is the separation constant.
(b) Assume that \(\lambda\) is real, and show that problem (ii) has no
nontrivial solutions if \(\lambda \leq 0\).
(c) If \(\lambda>0\), let \(\lambda=\mu^{2}\) with \(\mu>0 .\) Show that problem
(ii) has nontrivial solutions only if \(\mu\) is a solution of the equation
$$
\mu \cos \mu L+\gamma \sin \mu L=0
$$
(d) Rewrite Eq. (iii) as \(\tan \mu L=-\mu / \gamma .\) Then, by drawing the
graphs of \(y=\tan \mu L\) and \(y=-\mu L / \gamma L\) for \(\mu>0\) on the same set
of axes, show that Eq. (iii) is satisfied by infinitely many positive values
of \(\mu ;\) denote these by \(\mu_{1}, \mu_{2}, \ldots, \mu_{n}, \ldots,\)
ordered in increasing size.
(e) Determine the set of fundamental solutions \(u_{n}(x, t)\) corresponding to
the values \(\mu_{n}\) found in part (d).