Bad Sampling Triangle Wave = Amazing Pattern

coolpattern.py

import numpy as np
import scipy.io.wavfile as wavfile
import matplotlib.pyplot as plt

duration = 20
fs = 44100
y = np.zeros(fs*duration)
n_harmonics = 5
t = np.arange(fs*duration) # INCORRECT SAMPLING
#t = np.linspace(0, duration, fs*duration)  # CORRECT SAMPLING
freq = 440
for i in range(1, n_harmonics+1):
    coeff = (1.0/i)*(-1)**(i+1)
    y = y + coeff*np.sin(2*np.pi*freq*i*t)
y = y/np.max(np.abs(y))
plt.plot(y)
plt.show()
wavfile.write("out.wav", fs, y)