Calculus, constrained minimization and Lagrange multipliers: Is the optimal critical point a local minimizer?

Abstract

In this short note, we discuss how the optimality conditions for the problem of minimizing a multivariate function subject to equality constraints have been dealt with in undergraduate Calculus. We are particularly interested in the 2 or 3-dimensional cases, which are the most common cases in Calculus courses. Besides giving sufficient conditions to a critical point to be a local minimizer, we also present and discuss counterexamples to some statements encountered in the undergraduate literature on Lagrange Multipliers, such as `among the critical points, the ones which have the smallest image (under the function) are minimizers' or `a single critical point (which is a local minimizer) is a global minimizer'.