docs: public static void solveU( double U[] , double []b , int n )
      {
  //        for( int i =n-1; i>=0; i-- ) {
  //            double sum = b[i];
  //            for( int j = i+1; j <n; j++ ) {
  //                sum -= U[i*n+j]* b[j];
  //            }
  //            b[i] = sum/U[i*n+i];
  //        }
          for( int i =n-1; i>=0; i-- ) {
              double sum = b[i];
              int indexU = i*n+i+1;
              for( int j = i+1; j <n; j++ ) {
                  sum -= U[indexU++]* b[j];
              }
              b[i] = sum/U[i*n+i];
          }
      }