code: public Matrix getIminus() {
          Matrix Iminus = new Matrix(size, size);
          int i, j;
          for (i = 0; i < size; i++)
              for (j = 0; j < size; j++)
                  Iminus.matrix[i][j] = 0d;
          for (i = 1; i < size; i++)
              Iminus.matrix[i][i - 1] = Math.sqrt(J * J + J - (J - i) * (J - i) - (J - i));
          return Iminus;
      }
docs: Calculates the I- operator