MaksimenkoS. I. “Classification of<em class="a-Plus-plus">m</Em&gt;-Functions on Surfaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 8, Aug. 1999, pp. 1129–1135, https://umj.imath.kiev.ua/index.php/umj/article/view/4706.