Often, one omits the reference time, t , and reference radius, R , by assuming equilibrium (and thus time invariance of the ensemble) and averaging over all sample positions, yielding: C ( r ) = ⟨ ⟨ s 1 ( 0 ) ⋅ ⋅ s 2 ( r ) ⟩ ⟩ − − ⟨ ⟨ s 1 ( 0 ) ⟩ ⟩ ⟨ ⟨ s 2 ( r ) ⟩ ⟩ where, again, the choice of whether to subtract the uncorrelated variables differs among fields. The Radial distribution function is an example of an equal-time correlation function where the uncorrelated reference is generally not subtracted. Other equal-time spin-spin correlation functions are shown on this page for a variety of materials and conditions.