= First-order optimality condition
{title2=$\nabla f(x_*)=0$}
= First-order condition
{synonym}
At an interior local extremum of a <differentiable function>, its <gradient> is zero: restricting the function to every line through the point shows that all directional derivatives vanish. This is only a necessary condition in general. For a <strictly concave function>, an interior stationary point is the unique global maximizer on its convex domain. Constrained or boundary optima instead need conditions adapted to their feasible directions.
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