Suppose we have the two-loop integral ∫d4k2∫d4k1f(k1,k2), where k1 and k2 are four-dimensional vectors in Euclidean space. In the first integration with respect to k1, I take k2 to be the z-axis. Then k2=k1cosω and the four-dimensional spherical volume element is dV=|k1|3sin2ωsinθd|k1|dωdθdϕ, where |k1| is the Euclidean norm of k1.
When we perform the second integration ∫d4k2g(k2), where g(k2) is the result of the first integration, is it allowed to change coordinate systems and take k2 to be the radius in S3? That is, can we write the spherical volume element for the second integration as 2π2|k2|3, where |k2| is the Euclidean norm of k2?
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