Abstract
Zahra and Hikal recently proposed a nonstandard finite difference method for solving a class of variable-order fractional optimal control problems. They presented a system of variable-order fractional differential equations, and they claimed that its solution provides the optimal profiles for the state and control variables. In this note, we prove that this system does not give the necessary conditions of optimality for the variable-order fractional optimal control problems. Consequently, one cannot construct an indirect approach for the variable-order fractional optimal control problems using the Euler-Lagrange equations proposed by Zahra and Hikal.