An obstacle constraint requires a solution to lie above a given function. In an optimal stopping problem, the value dominates the payoff, satisfies a generator equation where continuation is optimal, and a generator inequality where stopping is optimal. The complementary conditions can be expressed as a maximum equal to zero.
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The "obstacle problem" typically refers to a type of variational problem in which one studies the properties of a function that satisfies certain conditions while being constrained by obstacles in its domain. More formally, it often pertains to finding the minimum of a functional subject to certain constraints represented by obstacles.