# Integrating a function forcing parameters to be real and positive

I'd like to integrate the function $f(x)=x^{a-1}e^{-b x}$ over the interval $[0,\infty)$.

f[x_] := x^(a - 1)*Exp[-b*x]

Then:

Integrate[f[x], {x, 0, \[Infinity]}]

But I get:

ConditionalExpression[b^-a Gamma[a], Re[b] > 0 && Re[a] > 0]

How can I force both a and b to be real, positive parameters, then try the integration again?

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Assuming[Re[b] > 0 && Re[a] > 0, Integrate[f[x], {x, 0, Infinity}]] Just copied what Integrate gave back into the Assuming part. –  Nasser Jun 22 '14 at 5:31
@Nasser looks like an answer... –  Yves Klett Jun 22 '14 at 5:48
It looks like a question asked instead of looking into documentation, does not it? –  Alexei Boulbitch Jun 23 '14 at 9:35