Probability of $N \ge n$ points for an inhomogeneous poisson point process

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I am trying to figure out the probability of at least n points for an inhomogeneous poisson point process defined on the real line.

$$ P\{N(a,\infty) \ge n \} = ? $$

I'm also not entirely sure if the following is valid (ie. integrating up to infinity):

$$ P\{N(a,\infty) = n \} = \frac{[\Lambda(a,\infty)]^n}{n!}e^{-\Lambda(a,\infty)} $$

where $\lambda(t)$ is some locally integrable positive function (for t>0)

$$ \Lambda(a,\infty) = \int_{a}^{\infty}\lambda(t)dt $$