Gabriel Р., L. A. Nazarova, A. V. Roiter, V. V. Sergeychuk, and D. Vossieck. 1993. “Tame and Wild Subspace Problems”. Ukrains’kyi Matematychnyi Zhurnal 45 (3): 313–352. https://umj.imath.kiev.ua/index.php/umj/article/view/5815.