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## Main Question or Discussion Point

I've been stuck over this integral for around an hour while studying the derivation of the second coefficient of the virial equation:

∫∫dx1d x2 γ(x1,x2) where γ(x1,x2) is 1 when x1 - x2 < constant.

= V∫ dx2 γ(x1,x2) where V is the integral of dx1.

Given: periodic boundary condition: x1 + V = x1

How do you justify this step?

∫∫dx1d x2 γ(x1,x2) where γ(x1,x2) is 1 when x1 - x2 < constant.

= V∫ dx2 γ(x1,x2) where V is the integral of dx1.

Given: periodic boundary condition: x1 + V = x1

How do you justify this step?