Abbasova, Y. G., and V. M. Kurbanov. “the Class $W_{p,m}^1 (G),\; P > 1$, in the Vector Eigenfunctions of a Differential Operator”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 6, June 2017, pp. 719-33, https://umj.imath.kiev.ua/index.php/umj/article/view/1731.