回复 1# lesley4373 的帖子
I will give a proof: partitial equilibrium: (局域均衡） 对不起用英文方便些：
the premises of partial equlibrium is that this market is very small that the fraction of the expenditure for the consumers on this goods is very small: or the income effect is negilible. so the utility function can be expressed as quasi-linear utility function : u(x,y) =v(x) +y s.t . px+y=m
first order condition (inner solution) v'=p, convexity means that v''<0 hence the marginal willing-ness to pay is diminish
