Akashi, Shigeo, and Tomofumi Matsuzawa. “A Version of Hilbert’s 13th Problem for Infinitely Differentiable Functions”. Results in Nonlinear Analysis 8, no. 2 (May 31, 2025): 72–75. Accessed June 7, 2025. https://nonlinear-analysis.com/index.php/pub/article/view/634.