I want to compute : $$\sum\_{n=-\infty}^{\infty}\delta(t-nT)$$
I tried as such:  

$$\cal{L} \Big( \sum\_{n=-\infty}^{\infty}\delta(t-nT) \Big)= \sum\_{n=-\infty}^{\infty}\cal{L}(\delta(t-nT)=\sum\_{n=-\infty}^{\infty} e^{-snT} = \sum\_{n=-\infty}^{0}e^{-snT} + \sum\_{n=0}^{\infty}e^{-snT}$$

The second one converges (geometric series) but the first one?