An adjoint variable is a mathematical concept used in optimization, particularly in control theory, that represents the sensitivity of the optimal value of a cost functional with respect to changes in the system state. These variables play a critical role in Pontryagin's minimum principle, as they help determine the optimal control laws by forming a system of equations that must be satisfied alongside the state dynamics. The adjoint variables are usually defined alongside a Hamiltonian function and provide insights into how changes in control inputs can affect system performance.
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