Gluon PDF constraints from the ratio of forward heavy-quark production at the LHC at and 13 TeVMatteo Cacciari, Michelangelo L. Mangano and Paolo NasonEur. Phys. J. C, 75 12 (2015) 610DOI: https://doi.org/10.1140/epjc/s10052-015-3814-x