In a post on the politics stack exchange, I came across the following description of how Marxism fits into the history of economic thought:
Most professional economists consider Marx' work on economic theory to be a substantial part of economic theory. Marx and Engels are included in the list of great classical economists, along with Adam Smith, Jeremy Bentham, David Ricardo, and Thomas Malthus. Marx pointed out that the long-term equilibrium for the "standard of living" is what a class insists upon before having, raising, and equipping enough children to replace itself. Several key concepts of economics (including "Cobb-Douglas production functions" and "value added") are designed to explain Marx' observations about the fraction of a factory's revenue that is not paid out for materials or proletarian labor.
The idea that "the long-term equilibrium for the 'standard of living' is what a class insists upon before having, raising, and equipping enough children to replace itself." seems foreign to me as an idea in economics and I am curious about the accuracy of the claims about Cobb Douglass and Value added, since they were not supported with references.
Is Marx commonly considered to be an influential classical economist, and if so, what is an example of an idea he added to economics as a discipline?