Affected by the availability of substitutes, degree of necessity, and time period over which it is calculated, isotropy can be achieved via the 'Arc' formulation of this economic quantity.  
The Marshall-Lerner condition says that the sum of this quantity for home imports and foreign imports must be greater than 1.  Affected by the availability of substitutes, degree of necessity, and time period over which it is calculated, isotropy can be achieved via the 'Arc' formulation of this economic quantity.  Typically negative, one exception is Giffen goods for which this value is by definition positive.  FTP, name this economic quantity which can be defined as the measure the responsiveness of the quantity demanded of a good to its price.
The Marshall-Lerner condition says that the sum of this quantity for home imports and foreign imports must be greater than 1.  
price elasticity of demand