  Finding approximate stationary points, i.e., points where the gradient is
approximately zero, of non-convex but smooth objective functions $f$ over
unrestricted $d$-dimensional domains is one of the most fundamental problems in
classical non-convex optimization. Nevertheless, the computational and query
complexity of this problem are still not well understood when the dimension $d$
of the problem is independent of the approximation error. In this paper, we
show the following computational and query complexity results:
  1. The problem of finding approximate stationary points over unrestricted
domains is PLS-complete.
  2. For $d = 2$, we provide a zero-order algorithm for finding
$\varepsilon$-approximate stationary points that requires at most
$O(1/\varepsilon)$ value queries to the objective function.
  3. We show that any algorithm needs at least $\Omega(1/\varepsilon)$ queries
to the objective function and/or its gradient to find $\varepsilon$-approximate
stationary points when $d=2$. Combined with the above, this characterizes the
query complexity of this problem to be $\Theta(1/\varepsilon)$.
  4. For $d = 2$, we provide a zero-order algorithm for finding
$\varepsilon$-KKT points in constrained optimization problems that requires at
most $O(1/\sqrt{\varepsilon})$ value queries to the objective function. This
closes the gap between the works of Bubeck and Mikulincer [2020] and Vavasis
[1993] and characterizes the query complexity of this problem to be
$\Theta(1/\sqrt{\varepsilon})$.
  5. Combining our results with the recent result of Fearnley et al. [2022], we
show that finding approximate KKT points in constrained optimization is
reducible to finding approximate stationary points in unconstrained
optimization but the converse is impossible.
