While it does not uniquely fix the form the Green's function will take, performing a dimensional analysis to find the units a Green's function must have is an important sanity check on any Green's function found through other means. A quick examination of the defining equation, L G ( x , s ) = δ δ ( x − − s ) , shows that the units of G depend not only on the units of L but also on the number and units of the space of which the position vectors x and s are elements. This leads to the relationship: [ [ G ] ] = [ [ L ] ] − − 1 [ [ d x ] ] − − 1 , where [ [ G ] ] is defined as, "the physical units of G ", and d x is the volume element of the space (or spacetime).