// derivee de mu par rapport a gp 
//
function Dmu = cal_Dmu(gp)
//
//  PE
   mu0 = 1.; lambda = 13,47; m = 0.294; a = .381
//   mu0 = 1;  lambda = 2.;  m = .2; a = 2.;

   zero = 0.0001;

   mu = cal_mu(gp);
   Dmu = mu*(m-1)*(lambda*gp)^a/(1+(lambda*gp)^a);
   
   if  gp >= zero then
        Dmu  = Dmu/gp;
      else
        Dmu = 0 ;
      end;

endfunction
