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Frisch demands are demand equations found usually in an intermediate step of the standard utility maximization problem. By example for the basic two-good case of a consumer with quasilinear preferences: $$U(x,y)=y+\ln (x)$$

the corresponding Frisch Demand is:

$$x^f(\lambda,\text{p})=\frac{1}{1+\lambda p_x}$$

In general what are Frish demands useful for on their own? What Questions do they answer?