#!/usr/bin/python
# 4.2

import scipy as sp
from pylab import *
a = 0.9
n = sp.arange(-10,21)
y = -(a**n*(n<=-1))
stem(n,y,markerfmt='.')
xlabel('Time(n)')
ylabel(r'$-(0.9)^n1(-n-1)$')
show()