calculating of dual variables

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If I want to calculate the values of a specific dual variable associated to a constraint in LP optimization problem without solving dual problem, is it possible to slightly change the Right Hand Side (RHS) of that constraint and calculate the sensitivity of the objective function? How much should I perturb the RHS of the constraint?

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Yes, you can change the right-hand slightly and re-solve the primal, then calculate the change in objective function. This will give you an estimate of the dual value. If you use a very tiny perturbation, you will run into numerical issues. If you use a non-trivial perturbation, you will run the risk that the basis changes and so the dual value no longer corresponds to the the change in objective function. In general there is no way to know in advance how much of a perturbation is the "right" amount.