euler condition ???

"Many economic problems are dynamic optimization problems
 in which choices are linked over time,
 for example
 a government deciding tax rates over time subject to an intertemporal budget constraint.

 Whatever solution approach one employs

 — the calculus of variations, optimal control theory or dynamic programming —

part of the solution is typically an Eulerequation

 which viewed as a condition of choice  optimality
  has this property:

any marginal, temporary and feasible change in behavior
 has marginal benefits equal to marginal costs in the present and future."

" Assuming the original problem satisfies regularity conditions,
 the Euler equation is a necessary but not sufficient condition for  an optimum. "

in other words

"An Euler equation is a difference or differential equation that is an intertemporal first-order condition
 for a dynamic choice problem.
It describes the evolution of economic variables along an optimal path.
 It is a necessary but not sufficient condition for a candidate optimal path,
 and so is useful for partially characterizing the theoretical implications
 of a range of models for dynamic behaviour"

that's about it ...or there abouts