Test function in functional analysis.

test.py

import numpy as np
import matplotlib.pylab as plt

def phi(x):
    y = np.zeros((len(x), ))
    index = np.absolute(x) < 1
    y[index] = np.exp(1/(x[index]**2-1))
    return y
    

x = np.linspace(-2, 2, 1000)
y = phi(x)

plt.plot(x, y)
plt.axis('equal')
plt.show()