Many mathematical economists assert that a set of functions in which Marx's equalities hold does "not" generally exist at the individual enterprise or aggregate level, so that chapter 9's transformation problem has no general solution, outside two very special cases. This was first pointed out by, among others, Böhm-Bawerk (1896) and Bortkiewicz (1906). In the second half of the 20th century, Leontief’s and Sraffa’s work on linear production models provided a framework within which to prove this result in a simple and general way.