code: protected DoubleMatrix1D gradSum(double t, DoubleMatrix1D fiX) {
  		DoubleMatrix1D gradSum = F1.make(getDim());
  		for(int i=0; i<dim; i++){
  			double d = 0;
  			d +=  1. / (t * fiX.getQuick(i));
  			d += -1. / (t * fiX.getQuick(getDim() + i));
  			gradSum.setQuick(i, d);
  		}
  		return gradSum;
  	}
docs: Calculates the second term of the first row of (11.55) "Convex Optimization".
  @see "Convex Optimization, 11.55"