Jódar, Lucas, and Almenar, Pedro. “Analytic-Numerical Solution With a Priori Error Bounds for Lateral Vibrations of a String With Variable Tension and Mass Density”. Utilitas Mathematica 55 (May 9, 1999). Accessed May 8, 2026. https://utilitasmathematica.com/index.php/Index/article/view/140.