Bondarenko, V. G. “Estimates of the Heat Kernel on a Manifold of Nonpositive Curvature”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 8 (August 25, 1998): 1129–1136. Accessed April 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4862.