诺斯在他的海洋运输生产率的文章中，有如下一段：
      Since ocean shipping during 1600-1860 was a highly competitive industry and on the industry level exhibited those features characteristic of  constant costs, the average freight rate would have tended to equal the cost per payload ton.6 Consequently, the index of total factor productivity can be written as：
                              A = Pi /C
     where A is total factor productivity, Pi is an index of input prices, and c is the freight rate index.


   各位，用指数法估计TFP，TFP=产出价格指数 / 投入价格指数。诺斯的写法怎么把投入价格指数作为分子呢？ 我的数学很差，谢谢！