HE, Kan, HOU, Jin Chuan, DOLINAR, Gregor and KUZMA, Bojan, 2011, Maps on self-adjoint operators preserving numerical range of products up to a factor. [online]. 2011. Vol. 54, no. 6, p. 925–932. [Accessed 3 April 2025]. Retrieved from: http://www.actamath.com/Jwk_sxxb_cn/CN/volumn/volumn_1986.shtml