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Notation: k = 1 , 2 , ..., d ; in 3. and 4, w$_{k}$ = E ( x$_{k}$f$_{z}$ ( z ) | z ); in 4a w = f$_{z}$$_{−}$$_{x}$ ; w$_{k}$ = E ( x$_{k}$w ( z − x ) | ( z − x )) . 

