1. The problem statement, all variables and given/known data Find laplace of e-atu(t) where a > 0 2. Relevant equations Laplace is integration from -inf to +inf f(t)e-stdt u(t) is 1 for t more than equal to 0. 3. The attempt at a solution Well i got the answer as X(s) = 1/(s+a). But the book said something like ROC like region of convergence and for that s must be more than -a I understand that s more than minus a, gives positive 1/(s+a) but why? Why can't ROC be negative? What does ROC mean? I am studying Signals and Processing.