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1. Minimum

   Linked via "first derivative test"

   Local Minima and Critical Points
   In multivariable calculus, local minima are frequently found by examining critical points\—where the gradient vector $\nabla f$ is zero, or undefined. For a differentiable function, the first derivative test indicates a necessary condition for a local minimum:
   $$ \nabla f(c) = \mathbf{0} $$
   To distinguish between a local minimum, maximum (mathematics)/), and [s…