(see Master Solution from the lecture assistants (VPN needed))
∂ ∂ x i H = − 2 ∑ j ( m i j − ⟨ x i , y j ⟩ ) ⋅ y j {\displaystyle {\frac {\partial }{\partial x_{i}}}H=-2\sum _{j}\left(m_{ij}-\langle x_{i},y_{j}\rangle \right)\cdot y_{j}}
∂ ∂ y j H = − 2 ∑ i ( m i j − ⟨ x i , y j ⟩ ) ⋅ x i {\displaystyle {\frac {\partial }{\partial y_{j}}}H=-2\sum _{i}\left(m_{ij}-\langle x_{i},y_{j}\rangle \right)\cdot x_{i}}
∂ H i j ∂ x i = − 2 ( m i j − ⟨ x i , y j ⟩ ) ⋅ y j {\displaystyle {\frac {\partial H_{ij}}{\partial x_{i}}}=-2\left(m_{ij}-\langle x_{i},y_{j}\rangle \right)\cdot y_{j}}
∂ H i j ∂ y j = − 2 ( m i j − ⟨ x i , y j ⟩ ) ⋅ x i {\displaystyle {\frac {\partial H_{ij}}{\partial y_{j}}}=-2\left(m_{ij}-\langle x_{i},y_{j}\rangle \right)\cdot x_{i}}