Cosmological investigation of multi-frequency VLBI observations of ultra-compact structure in radio quasarsShuo Cao, Marek Biesiada, Jingzhao Qi, Yu Pan, Xiaogang Zheng, Tengpeng Xu, Xuan Ji and Zong-Hong ZhuEur. Phys. J. C, 78 9 (2018) 749DOI: https://doi.org/10.1140/epjc/s10052-018-6197-y