diff --git a/Book4_Ch19_Python_Codes/Bk4_Ch19_01.py b/Book4_Ch19_Python_Codes/Bk4_Ch19_01.py
deleted file mode 100644
index 8d8e7c1..0000000
--- a/Book4_Ch19_Python_Codes/Bk4_Ch19_01.py
+++ /dev/null
@@ -1,78 +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_Ch19_01.py
-
-import sympy
-
-#define symbolic vars, function
-x1,x2 = sympy.symbols('x1 x2')
-# f_x = 3*x1**2-5*x2**2
-f_x = x1 - x2
-
-#take the gradient symbolically
-grad_f = [sympy.diff(f_x,var) for var in (x1,x2)]
-
-f_x_fcn = sympy.lambdify([x1,x2],f_x)
-
-#turn into a bivariate lambda for numpy
-grad_fcn = sympy.lambdify([x1,x2],grad_f)
-
-import numpy as np
-import matplotlib.pyplot as plt
-
-xx1, xx2 = np.meshgrid(np.linspace(-4,4,41),np.linspace(-4,4,41))
-
-# coarse mesh
-xx1_, xx2_ = np.meshgrid(np.linspace(-4,4,10),np.linspace(-4,4,10))
-V = grad_fcn(xx1_,xx2_)
-V_z = np.ones_like(V[1]);
-
-ff_x = f_x_fcn(xx1,xx2)
-# ff_x_ = f_x_fcn(xx1_,xx2_)
-
-color_array = np.sqrt(V[0]**2 + V[1]**2)
-l_3D_vectors = np.sqrt(V[0]**2 + V[1]**2 + V_z**2)
-
-# 3D 
-ax = plt.figure().add_subplot(projection='3d')
-ax.plot_wireframe(xx1, xx2, ff_x, rstride=1, 
-                  cstride=1, color = [0.5,0.5,0.5],
-                  linewidth = 0.2)
-ax.contour3D(xx1, xx2, ff_x,20)
-ax.contour3D(xx1, xx2, ff_x, levels = 0, colors = 'k')
-
-# plt.quiver (xx1_, xx2_, ff_x_, V[0], V[1], V_z,length = .5)
-ax.xaxis.set_ticks([])
-ax.yaxis.set_ticks([])
-ax.zaxis.set_ticks([])
-plt.xlim(-4,4)
-plt.ylim(-4,4)
-ax.view_init(30, -125)
-ax.set_xlabel('$x_1$')
-ax.set_ylabel('$x_2$')
-ax.set_zlabel('$f(x_1,x_2)$')
-plt.tight_layout()
-
-
-# 2D
-fig, ax = plt.subplots()
-plt.quiver (xx1_, xx2_, V[0], V[1], color_array,
-            angles='xy', scale_units='xy',scale=2,
-            edgecolor='none', facecolor= 'b')
-
-plt.contour(xx1, xx2, ff_x,20)
-plt.contour(xx1, xx2, ff_x, levels = 0, colors = 'k')
-plt.show()
-ax.set_aspect('equal')
-ax.xaxis.set_ticks([])
-ax.yaxis.set_ticks([])
-ax.set_xlabel('$x_1$')
-ax.set_ylabel('$x_2$')
-plt.tight_layout()