Differentiating p.x=w with respect to pk yields ∑p 2x;(P,v) +xk=0 opk or multiplying the whole expression by ph Sx(p-")B+②=∑m+mk=0 opk where again ni pixiDifferentiating p  x  w with respect to pk yields:  i1 L pi xip,w pk  xk  0 or multiplying the whole expression by pk w :  i1 L xip,w pk pk xi pixi w  pkxk w   i1 L pk i i  k  0, where again i  pixi w