Inverse Laplace Transform of $\dfrac{1}{s^a+b}$

Question

I could not find on tables, but trying some values I think that is something with $ ERFC$ functions.

I am seeking for a general formula to the inverse Laplace transform of $\dfrac{1}{s^a+b},$ where $a$ and $b$ are positive.

Someone know this formula?

Thank you so much.

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Please, see comments. Do you have any ideas to show at least that the inverse is positive? Thank you!