Academic studies suggest that it would be extremely costly for market participants to attain and assimilate perfectly new information. In his book, A Theory of Adaptive Economic Behavior, Cross (1983) discusses the costliness of solving the complex statistical problems that modern economic and financial theory assume that individuals in the market are working. The methodological price for this approach [traditional statistical and mathematical decision analysis] has been extremely high, however, for it has become necessary toassume individuals in these markets can be represented as mathematical statisticianscapable of solving specific problems that are often beyond the analytic abilities of professionals in that field.(Technical analysis: the complete resource for financial market technicians / Charles D. Kirkpatrick and Julie Dahlquist.) I can't fully understand the bit in bold. What especially is nagging at me is the verb 'to work' here. It, in my humble opinion, doesn't completely fit with 'for it has become necessary to assume individuals in these markets can be represented as mathematical statisticians capable of solving specific problems'. I would consent to at least '... the complex statistical problems... that individuals in the market are working on.', although I'm not sure of its perfect suitability here as well. Would you be so kind to explain it to me? (I'm also slightly worried about the word 'assume'. Shouldn't it by any chance be 'assumes'?) Thanks.

If the individuals are working (correctly), then they are solving problems. Your plan to drive to the city assumes that the car is working. (X and Y assume. "Economic and financial theory assume" is "Economic theory and financial theory assume.")

I notice that 'working to solve (something)' is a common collocate, with some 127,000 hits on Google Books alone.

I agree with your objections. If the author meant individuals work on problems he'd have used on which (individuals work) instead of that. Still, we can say work (a math problem) to mean solve, so it isn't exactly wrong, especially since he used solve earlier in the text. Nevertheless I'd have written it on which, and I think that's what you'd prefer, too. If he'd written economic theory and financial theory, or economic and financial theories, assume would be proper, but like you I think the way it was actually written is strange. I agree with everybody about what it's intended to mean.