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## Main Question or Discussion Point

I am reading a book about integration on all possible momentum in 3D space, and it change the integration to a 1D integration and integration on solid angle, I don't know how does it happen

[tex]

\int d^3p \to \int p^2dp\int d\Omega

[/tex]

where [tex]\Omega[/tex] is the solid angle.

[tex]

\int d^3p \to \int p^2dp\int d\Omega

[/tex]

where [tex]\Omega[/tex] is the solid angle.