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Quadratic#
In this example we consider the proximal operator for a quadratic function:
\[\frac{1}{2} \mathbf{x}^T \mathbf{Op} \mathbf{x} + \mathbf{b}^T
\mathbf{x} + c.\]
which is implemented by the pyproximal.Quadratic
class.
import numpy as np
import matplotlib.pyplot as plt
import pylops
import pyproximal
plt.close('all')
To start with cosider the most complete case when both \(\mathbf{Op}\) and \(\mathbf{Op}\) are non-null.
x = np.arange(-5, 5, 0.1)
nx = len(x)
A = np.random.normal(0, 1, (nx, nx))
A = A.T @ A
c = 2.
quad = pyproximal.Quadratic(Op=pylops.MatrixMult(A), b=np.ones_like(x), c=c,
niter=500)
print('1/2 x^T Op x + b^T x + c: ', quad(x))
tau = 4
xp = quad.prox(x, tau)
xdp = quad.proxdual(x, tau)
plt.figure(figsize=(7, 2))
plt.plot(x, x, 'k', lw=2, label='x')
plt.plot(x, xp, 'r', lw=2, label='prox(x)')
plt.plot(x, xdp, 'b', lw=2, label='dualprox(x)')
plt.xlabel('x')
plt.title(r'$\frac{1}{2} \mathbf{x}^T \mathbf{Op} \mathbf{x} + '
r'\mathbf{b}^T \mathbf{x} + c$')
plt.legend()
plt.tight_layout()

1/2 x^T Op x + b^T x + c: 41035.02307854726
If we now assume that the operator \(\mathbf{Op}\) is null, the quadratic operator can be used to define the dot-product between \(\mathbf{x}\) and a vector \(\mathbf{b}\)
x = np.arange(-5, 5, 0.1)
dot = pyproximal.Quadratic(b=np.ones_like(x))
print('b^T x: ', quad(x))
tau = 2
xp = dot.prox(x, tau)
xdp = dot.proxdual(x, tau)
plt.figure(figsize=(7, 2))
plt.plot(x, x, 'k', lw=2, label='x')
plt.plot(x, xp, 'r', lw=2, label='prox(x)')
plt.plot(x, xdp, 'b', lw=2, label='dualprox(x)')
plt.xlabel('x')
plt.title(r'$\mathbf{b}^T \mathbf{x}$')
plt.legend()
plt.tight_layout()

b^T x: 41035.02307854726
Finally if also \(\mathbf{b}\) is zero, the quadratic function reduces to a constant \(\mathbf{c}\) and its proximity operator becomes the vector \(\mathbf{x}\) itself.
x = np.arange(-5, 5, 0.1)
dot = pyproximal.Quadratic(c=5.)
print('c: ', quad(x))
tau = 2
xp = dot.prox(x, tau)
xdp = dot.proxdual(x, tau)
plt.figure(figsize=(7, 2))
plt.plot(x, x, 'k', lw=2, label='x')
plt.plot(x, xp, 'r', lw=2, label='prox(x)')
plt.plot(x, xdp, 'b', lw=2, label='dualprox(x)')
plt.xlabel('x')
plt.title(r'$c$')
plt.legend()
plt.tight_layout()

c: 41035.02307854726
Total running time of the script: (0 minutes 0.423 seconds)