∇ ∇ ⋅ ⋅ ( ∇ ∇ v ) = ∇ ∇ ⋅ ⋅ ( v i , j e i ⊗ ⊗ e j ) = v i , j i e i ⋅ ⋅ e i ⊗ ⊗ e j = ( ∇ ∇ ⋅ ⋅ v ) , j e j = ∇ ∇ ( ∇ ∇ ⋅ ⋅ v ) ∇ ∇ ⋅ ⋅ [ ( ∇ ∇ v ) T ] = ∇ ∇ ⋅ ⋅ ( v j , i e i ⊗ ⊗ e j ) = v j , i i e i ⋅ ⋅ e i ⊗ ⊗ e j = ∇ ∇ 2 v j e j = ∇ ∇ 2 v