We usually use \mathcal{L} (rendered ) to denote the Laplace Transform. Similarly, \mathcal{L}^{-1} (rendered ) is used to denote the Inverse Laplace transform.

So for example,

and

I hope this helps!

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P.S.: Since LaTeX on MHF isn't working properly, we've come up with an alternative way to have the images rendered. So see this to find out more.