TF1 *pdf_xgx = new TF1("pdf_xgx", "[0]+[1]*log10(x) + [2]*(log10(x))**2 + [3]*(log10(x))**3");

GRV98 PDF, x*g(x):

Q=4GeV: (for charms)
  EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  p0          -1.55996e-01   9.94084e-02   2.97733e-05   9.85947e-07
   2  p1          -6.83198e-01   3.65323e-01   4.29559e-05  -4.90706e-07
   3  p2           4.77242e-01   3.13936e-01   2.67849e-05   2.68904e-07
   4  p3          -2.43645e-01   6.65177e-02   1.00684e-05   1.57518e-07


Q=10GeV: (for beauty)


 EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  p0          -1.71476e-01   9.94084e-02   4.21497e-05  -6.71618e-07
   2  p1          -7.67701e-01   3.65323e-01   6.08122e-05   5.82239e-07
   3  p2          -8.90736e-02   3.13936e-01   3.79191e-05  -8.76781e-07
   4  p3          -6.85866e-01   6.65177e-02   1.42537e-05   1.92015e-06