# Gauss-Newton¶

In [8]:
import numpy as np
import scipy as sp
import matplotlib.pyplot as pt
import scipy.linalg as la


We would like to fit the model $f(t) = x_0 e^{x_1 t}$ to the following data using Gauss-Newton:

In [13]:
t = np.array([0.0, 1.0, 2.0, 3.0])
y = np.array([2.0, 0.7, 0.3, 0.1])


First, define a residual function (as a function of $\mathbf x=(x_0, x_1)$)

In [14]:
def residual(x):
return y - x[0] * np.exp(x[1] * t)


Next, define its Jacobian matrix:

In [15]:
def jacobian(x):
return np.array([
-np.exp(x[1] * t),
-x[0] * t * np.exp(x[1] * t)
]).T


Here are two initial guesses. Try both:

In [56]:
#x = np.array([1, 0])
x = np.array([0.4, 2])


Here's a plotting function to judge the quality of our guess:

In [57]:
def plot_guess(x):
pt.plot(t, y, 'ro', markersize=20, clip_on=False)
T = np.linspace(t.min(), t.max(), 100)
Y = x[0] * np.exp(x[1] * T)
pt.plot(T, Y, 'b-')

print("Residual norm:", la.norm(residual(x), 2))

plot_guess(x)

Residual norm: 162.72704101675862


Code up one iteration of Gauss-Newton. Use numpy.linalg.lstsq() to solve the least-squares problem, noting that that function returns a tuple--the first entry of which is the desired solution.

Also print the residual norm. Use plot_iterate to visualize the current guess.

Then evaluate this cell in-place many times (Ctrl-Enter):

In [79]:
x = x + la.lstsq(jacobian(x), -residual(x))[0]

plot_guess(x)

Residual norm: 0.044677532987197045

In [ ]: