(a12+…+ar2)12⁢nn!⁢Hn⁡(a1⁢x1+⋯+ar⁢xr(a12+⋯+ar2)12)=∑m1+⋯+mr=na1m1⁢⋯⁢armrm1!⁢⋯⁢mr!⁢Hm1⁡(x1)⁢⋯⁢Hmr⁡(xr).