Bar problem and heat conduction equation A thin bar, defined through $ x\in [0,l] $ has a temparature distribution $ \theta (x,t) $
and has at the point $ x=0 $ a temperature of $0$ .
At the other end there is a heat emission to another medium of temperature $0$.
Here holds $$ \frac{ \partial \theta }{ \partial x} (l,t)+ \sigma \theta (l,t)= 0 $$ for all $ t \geq 0 $
At the timepoint $ t_o =0 $ the bar has a temperature destribution $ x \mapsto f(x) $
And it holds the heat conduction equation
$$ \frac{ \partial \theta }{ \partial t} = a^2 \frac{ \partial^2 \theta }{ \partial x^2} $$
where $ \sigma \in \mathbb{ R}^+, a \in \mathbb{R} \backslash \{0\} $ are constants.
Firstly..how can I show with the seperation method , so plugging in $ \theta (x,t)= u(x)v(t) $
I am confused, at which equation do I have to use the separation method?
that it will become the Sturm-Liouville Eigenvalueproblem
$$ u''+ \lambda u = 0  , u(0)=0$$
$$ \sigma u(l)+ u'(l)=0 $$
where $ \lambda \in \mathbb{R} $ is a constant.
And how can I solve the problem and determine the Eigenvalues?
many thanks in advance!
 A: Let us consider a solution of type $\theta(x,t)=u(x)v(t)$, then substituting into the equation we have
$$
u(x)v'(t)=a^2u''(x)v(t)\quad\implies\quad a^2\frac{u''(x)}{u(x)}=\frac{v'(t)}{v(t)}=-\lambda^2,
$$
so that, for $\lambda>0,$
\begin{align}
u(x) &= c_1\cos\left(\frac{\lambda}{a}x\right)+c_2\sin\left(\frac{\lambda}{a}x\right),\\
v(t) &= d_1e^{-\lambda^2t},
\end{align}
while for $\lambda=0$ we have
\begin{align}
u(x) &= c_1+c_2x,\\
v(t) &= d_1,
\end{align}
The condition $\theta(0,t)=0$ gives $u(0)=0$ so that $c_1=0$, while the condition $\partial\theta/\partial x+\sigma\theta|_{x=l}=0$ gives
$$
\frac{\lambda}{a}\cos\left(\frac{\lambda}{a}l\right)+\sigma\sin\left(\frac{\lambda}{a}l\right)=0
$$
if we set $x=\lambda l/a$, this can be recast as $\tan x=-\mu x$, where $\mu=l/\sigma$, whose solution cannot be obtained exactly (see graph), but provides an infinite number of solutions $\lambda_k$, $k=0,1,2,\ldots$.

So the general solution is
$$
\theta(x,t)=c_0x+\sum_{k=1}^\infty c_k\sin\left(\frac{\lambda_k}{a}x\right)e^{-\lambda_k^2t}
$$
It remains the initial condition
$$
\theta(x,0)=c_0x+\sum_{k=1}^\infty c_k\sin\left(\frac{\lambda_k}{a}x\right)=f(x),\quad x\in[0,l].
$$
When the $\lambda_k$ are multiple of integers, one can use Fourier series to obtain the $c_k$, but in this case I don't know how to proceed.
