1. The problem statement, all variables and given/known data I need to integrate this expression : P(k, w) = A * δ(w-k*v) * f(k, w) A is constant and δ, Dirac Delta. 2. Relevant equations There is double integration : I = ∫0∞ dk ∫0∞ P(k,w) dw = A ∫∫0∞ δ(w-k*v) * f(k, w) dw dk 3. The attempt at a solution I'm confused with Delta Dirac for calculating this integral. Let us proceed the w-variable integration firstly. Since the δ-term impose w = k*v, we need to calculate : ∫0∞ f(k=w/v, w) dw But after that, there is no longer any k-dependence... Of course I'm wrong.. can you explain me how to proceed ?? Thanks in advance !