diff --git a/Book4_Ch12_Python_Codes/Bk4_Ch12_01.py b/Book4_Ch12_Python_Codes/Bk4_Ch12_01.py
deleted file mode 100644
index 3908794..0000000
--- a/Book4_Ch12_Python_Codes/Bk4_Ch12_01.py
+++ /dev/null
@@ -1,54 +0,0 @@
-
-###############
-# Authored by Weisheng Jiang
-# Book 4  |  From Basic Arithmetic to Machine Learning
-# Published and copyrighted by Tsinghua University Press
-# Beijing, China, 2022
-###############
-
-# Bk4_Ch12_01.py
-
-import numpy as np
-from matplotlib import pyplot as plt
-
-x1 = np.arange(-2,2,0.05)
-x2 = np.arange(-2,2,0.05)
-
-xx1_fine, xx2_fine = np.meshgrid(x1,x2)
-
-a = 0; b = -1; c = 0;
-
-yy_fine = a*xx1_fine**2 + 2*b*xx1_fine*xx2_fine + c*xx2_fine**2
-
-# 3D visualization
-
-fig, ax = plt.subplots()
-ax = plt.axes(projection='3d')
-
-ax.plot_wireframe(xx1_fine,xx2_fine,yy_fine,
-                  color = [0.8,0.8,0.8],
-                  linewidth = 0.25)
-
-ax.contour3D(xx1_fine,xx2_fine,yy_fine,15,
-             cmap = 'RdYlBu_r')
-
-ax.view_init(elev=30, azim=60)
-ax.xaxis.set_ticks([])
-ax.yaxis.set_ticks([])
-ax.zaxis.set_ticks([])
-plt.tight_layout()
-ax.set_proj_type('ortho')
-plt.show()
-
-# 2D visualization
-
-fig, ax = plt.subplots()
-
-ax.contourf(xx1_fine,xx2_fine,yy_fine,15,
-           cmap = 'RdYlBu_r')
-
-ax.xaxis.set_ticks([])
-ax.yaxis.set_ticks([])
-ax.set_aspect('equal')
-
-plt.show()
diff --git a/Book4_Ch12_Python_Codes/Bk4_Ch12_02.py b/Book4_Ch12_Python_Codes/Bk4_Ch12_02.py
deleted file mode 100644
index 21edd33..0000000
--- a/Book4_Ch12_Python_Codes/Bk4_Ch12_02.py
+++ /dev/null
@@ -1,56 +0,0 @@
-
-###############
-# Authored by Weisheng Jiang
-# Book 4  |  From Basic Arithmetic to Machine Learning
-# Published and copyrighted by Tsinghua University Press
-# Beijing, China, 2022
-###############
-
-# Bk4_Ch12_02.py
-
-import matplotlib.pyplot as plt
-from mpl_toolkits.mplot3d import Axes3D
-import numpy as np
-import math as m
-
-cos_theta_12 = np.cos(m.radians(135))
-cos_theta_13 = np.cos(m.radians(60))
-cos_theta_23 = np.cos(m.radians(120))
-
-P = np.array([[1, cos_theta_12, cos_theta_13],
-              [cos_theta_12, 1, cos_theta_23],
-              [cos_theta_13, cos_theta_23, 1]])
-
-L = np.linalg.cholesky(P)
-R = L.T
-
-
-fig = plt.figure()
-ax = fig.add_subplot(projection='3d')
-
-plt.plot(0,0,0,color = 'r', marker = 'x',
-         markersize = 12)
-
-colors = ['b', 'r', 'g']
-for i in np.arange(0,3):
-    
-    vector = R[:,i]
-    v = np.array([vector[0],vector[1],vector[2]])
-    vlength=np.linalg.norm(v)
-    ax.quiver(0,0,0,vector[0],vector[1],vector[2],
-            length=vlength, color = colors[i])
-
-ax.set_xlim([-1,1])
-ax.set_ylim([-1,1])
-ax.set_zlim([-1,1])
-
-ax.set_xlabel('x')
-ax.set_ylabel('y')
-ax.set_zlabel('z')
-
-ax.xaxis.set_ticklabels([])
-ax.yaxis.set_ticklabels([])
-ax.zaxis.set_ticklabels([])
-ax.view_init(35, 60)
-ax.set_proj_type('ortho')
-ax.set_box_aspect([1,1,1])