One of Fermat's theorems states that
Optima of unconstrained problems are found at stationary points,
• where the 1st derivative or the gradient of the objective function is zero
– More generally, they may be found at critical points,
• where the 1st derivative or gradient of the objective function is zero or undefined , or on the boundary of the choice set.
Optima of inequality-constrained problems are instead
– found on the boundary of the constrained set or found by the gradient values using Lagrange multiplier method.