Existence of solution for a Langevin equation involving the $ \psi $-Hilfer fractional derivative: A variational approach

<p>This study examines the existence of a solution for a nonvariational Langevin equation that involves the $ \psi $-Hilfer fractional derivative. More specifically, we apply the mountain pass theorem, and then an iterative approach to establish the existence of a solution for the problem.</p>