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()