Cicognani, Massimo, and Fumihiko Hirosawa. “On the Gevrey Well-Posedness for Second Order Strictly Hyperbolic Cauchy Problems under the Influence of the Regularity of the Coefficients”. Mathematica Scandinavica 102, no. 2 (June 1, 2008): 283–304. Accessed July 24, 2026. https://journals.msp.org/mscand/article/view/634.