BondarenkoV. M., and StepochkinaM. V. 2009. “Description of Posets Critical With Respect to the Nonnegativity of the Quadratic Tits Form”. Ukrains’kyi Matematychnyi Zhurnal 61 (5), 611-24. https://umj.imath.kiev.ua/index.php/umj/article/view/3045.