1 #!/usr/bin/python

     2 # 4.1

     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>=0)

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

    10 xlabel('Time(n)')

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

    12 

    13 show()