Existence of solutions and Hyers-Ulam stability for a coupled system of nonlinear Langevin equations involving φ-Riemann-Liouville and ψ-Caputo fractional derivatives subject to mixed boundary conditions. GJOM [Internet]. 2025 Aug. 18 [cited 2026 Sep. 22];20(2):489-508. Available from: https://gjom.org/index.php/gjom/article/view/3279