Matrix norms

In [1]: 
import numpy as np
import numpy.linalg as la
import matplotlib.pyplot as pt

Here's a matrix of which we're trying to compute the norm:

In [14]: 
n = 2
A = np.random.randn(n, n)

Recall:

$$||A||=\max_{\|x\|=1} \|Ax\|,$$

where the vector norm must be specified, and the value of the matrix norm $\|A\|$ depends on the choice of vector norm.

For instance, for the $p$-norms, we often write:

$$||A||_2=\max_{\|x\|=1} \|Ax\|_2,$$

and similarly for different values of $p$.

We can approximate this by just producing very many random vectors and evaluating the formula:

In [16]: 
xs = np.random.randn(n, 1000)

First, we need to bring all those vectors to have norm 1. First, compute the norms:

In [32]: 
p = 2
norm_xs = np.sum(np.abs(xs)**p, axis=0)**(1/p)
norm_xs.shape
Out[32]:
(1000,)

Then, divide by the norms and assign to normalized_xs:

In [33]: 
normalized_xs = xs/norm_xs
la.norm(normalized_xs[:, 316], p)
Out[33]:
1.0

Let's take a look:

In [34]: 
pt.plot(normalized_xs[0], normalized_xs[1], "o")
pt.gca().set_aspect("equal")

Now apply $A$ to these normalized vectors:

In [35]: 
A_nxs = A.dot(normalized_xs)

Let's take a look again:

In [36]: 
pt.plot(normalized_xs[0], normalized_xs[1], "o", label="x")
pt.plot(A_nxs[0], A_nxs[1], "o", label="Ax")
pt.legend()
pt.gca().set_aspect("equal")

Next, compute norms of the $Ax$ vectors:

In []: 
norm_Axs = np.sum(np.abs(A_nxs)**p, axis=0)**(1/p)
norm_Axs.shape

What's the biggest one?

In [23]: 
np.max(norm_Axs)
Out[23]:
2.3888099889735472

Compare that with what numpy thinks the matrix norm is:

In [24]: 
la.norm(A, p)
Out[24]:
2.3888099931809323
In [9]: 
A = np.arange(9).reshape(3,3)
A
Out[9]:
array([[0, 1, 2],
       [3, 4, 5],
       [6, 7, 8]])
In [13]: 
np.sum(A)
Out[13]:
36
In []: