Semileptonic decays of meson to S-wave charmonium states in the perturbative QCD approachZhou Rui, Hong Li, Guang-xin Wang and Ying XiaoEur. Phys. J. C, 76 10 (2016) 564DOI: https://doi.org/10.1140/epjc/s10052-016-4424-y