https://root.cern.ch/doc/master/classROOT_1_1Fit_1_1FitUtil_1_1SimpleGradientCalculator.html
ROOT: ROOT::Fit::FitUtil::SimpleGradientCalculator Class Reference
rootfitclassreference
https://root.cern.ch/doc/v614/classROOT_1_1Fit_1_1FitUtil_1_1LikelihoodAux_3_01double_01_4.html
ROOT: ROOT::Fit::FitUtil::LikelihoodAux double Class Template Reference
rootfitdoubleclasstemplate