where λ λ is the damping constant. The term η η ( t ) is white noise, characterized by ⟨ η η ( t ) η η ( t ′ ) ⟩ = 2 k B T λ λ δ δ ( t − − t ′ ) (formally, the Wiener process). One way to solve this equation is to introduce a test function f and calculate its average. The average of f ( x ( t ) ) should be time-independent for finite x ( t ) , leading to