Substituting the relation r − − ℓ ℓ | r − − ℓ ℓ | 3 = − − ∇ ∇ ( 1 | r − − ℓ ℓ | ) and using the product rule for curls, as well as the fact that J does not depend on r , this equation can be rewritten as B ( r ) = μ μ 0 4 π π ∇ ∇ × × ∭ ∭ V d 3 ℓ ℓ J ( ℓ ℓ ) | r − − ℓ ℓ |