∂ j ⁡ R − a ⁡ ( b ; z ) = − a ⁢ w j ⁢ R − a − 1 ⁡ ( b + e j ; z ) \partial _{j}R_{-a}\left(\mathbf {b} ;\mathbf {z} \right)=-aw_{j}R_{-a-1}\left(\mathbf {b} +\mathbf {e} _{j};\mathbf {z} \right)