Theorien über den Mehrwert
Marxian economics has been built upon by many others, beginning almost at the moment of Marx's death. The second and third volumes of "Das Kapital" were edited by his close associate Friedrich Engels, based on Marx's notes. Marx's "Theories of Surplus Value" was edited by Karl Kautsky. The Marxian value theory and the Perron–Frobenius theorem on the positive eigenvector of a positive matrix are fundamental to mathematical treatments of Marxian economics. The relation between exploitation (surplus labour) and profit has been modeled with increased sophistication.