docs: public double[][] diff_yy(double matyy[][]) {
          mat3 = new double[width][height];
          double mat1, mat2;
          for (int i = 0; i < width; i++) {
              for (int j = 0; j < height; j++) {
                  if (i == 0) {
                      mat1 = matyy[i][j];
                      mat2 = matyy[i + 1][j];
                  } else if (i == width - 1) {
                      mat1 = matyy[i - 1][j];
                      mat2 = matyy[i][j];
                  } else {
                      mat1 = matyy[i - 1][j];
                      mat2 = matyy[i + 1][j];
                  }
                  mat3[i][j] = (mat1 + mat2 - 2 * matyy[i][j]);
              }
          }
          return mat3;
  
      }