We substitute this into equation (2) and equate the coefficients belonging to the same frequency, keeping only the lowest-order terms. As we show below, h 0 and h 2 are smaller than h 1 , being of order h 1 2 . h 3 , and all further coefficients, are at least of order h 1 3 . This makes sense, since h 0 , h 2 , h 3 , … … must all vanish faster than h 1 as a circular orbit is approached.