Aslanova, N. M., and M. Bairamogly. “On Generalized Regularized Trace of a Fourth-Order Differential Operator With Operator Coefficient”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 1, Jan. 2014, pp. 128–134, https://umj.imath.kiev.ua/index.php/umj/article/view/2116.