Burtnyak, I. V., and H. P. Malyts’ka. 2018. “On the Fundamental Solution of the Cauchy Problem for Kolmogorov Systems of the Second Order”. Ukrains’kyi Matematychnyi Zhurnal 70 (8), 1107-17. https://umj.imath.kiev.ua/index.php/umj/article/view/1621.