**4. Conclusions**

In this article, on the integral boundary value problem for conformable fractional differential equations, we use the monotone iterative technique to investigate the existence results for extremal solutions for Equation (1). At the same time, two sequences are obtained using the upper and lower solutions, and these two sequences approximate the extremal solutions of nonlinear differential equations. It is clear that the method of using the upper and lower solutions is a very effective method for studying the solvability of conformable fractional differential equations. However, almost all the results derived in the paper are more-or-less straightforward extensions of well-known results from the theory of the first-order differential equations, since the conformal fractional derivative is essentially a modified version of the first-order derivative.

**Author Contributions:** The authors have made the same contribution.

**Funding:** This work was partially supported by the Natural Science Foundation of China (11371221, 11571207), the Shandong Natural Science Foundation (ZR2018MA011), and the Tai'shan Scholar Engineering Construction Fund of Shandong Province of China.

**Conflicts of Interest:** The authors declare that they have no competing interests.
