It's against the law. It's an illegal contract.

因为那是违法的,这是个无效合同

耶鲁公开课 - 博弈论课程节选

It's illegal, so they can't sign that as a contract, but they they can just agree to do it.

因为是违法的,合同也就无效了,不过他们可以私下达成默契

耶鲁公开课 - 博弈论课程节选

It's illegal to sign an agreement, to sign a contract saying you're going to restrict quantities.

签订这样限制产量的协议,属于违法行为

耶鲁公开课 - 博弈论课程节选