For example, if L = ∂ ∂ t 2 and time is the only variable then: [ [ L ] ] = [ [ time ] ] − − 2 , [ [ d x ] ] = [ [ time ] ] , and [ [ G ] ] = [ [ time ] ] . If L = ◻ ◻ = 1 c 2 ∂ ∂ t 2 − − ∇ ∇ 2 , the d'Alembert operator, and space has 3 dimensions then: [ [ L ] ] = [ [ length ] ] − − 2 , [ [ d x ] ] = [ [ time ] ] [ [ length ] ] 3 , and [ [ G ] ] = [ [ time ] ] − − 1 [ [ length ] ] − − 1 .