L ∑ axi(p, w) wpix Xi w ∑:1 Where ni pix the budget share of good This means that income elasticities(when weighted by budget shares)sum to one All goods cant be luxuries, etc Sometimes this is known as engel aggregation(income effects are after all drawn with Engel curves) i1 L xip,w w w xi pixi w   i1 L w i i  0, where i  pixi w , the budget share of good i. This means that income elasticities (when weighted by budget shares) sum to one. All goods can’t be luxuries, etc. Sometimes this is known as Engel aggregation (income effects are after all drawn with Engel curves)