Solve the PDE by Separation of variables method.

$\{\begin{array}{ccc}& \text{P}\text{D}\text{E}\text{:}\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{t}}={}^2[\frac{de{l}^{2}u}{del{r}^2}+\frac{1}{r}\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{r}}],& t>\end{array}$

Bis: $\{\begin{array}{ccc}\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{r}}=0& at& \text{r}\text{z}\text{a}\end{array}$

$u=0at$ rza

$Ic$: $u(r,0)=3,ra$