Nielsen[i,j, x] denotes Nielsen's polylogarithm.
See also: SimplifyPolyLog.
Numerical evaluation is done via N[Nielsen[n_,p_,x_]] := (-1)^(n+p-1)/(n-1)!/p! NIntegrate[Log[1-x t]^p Log[t]^(n-1)/t,{t,0,1}];
Some special values are built in.
The FeynCalc Book | MTD | NTerms |