1 #!/usr/bin/python

     2 # 4.2

     3 

     4 import scipy as sp

     5 from pylab import *

     6 a = 0.9

     7 n = sp.arange(-10,21)

     8 y = -(a**n*(n<=-1))

     9 stem(n,y,markerfmt='.')

    10 xlabel('Time(n)')

    11 ylabel(r'$-(0.9)^n1(-n-1)$')

    12 show()