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#!/usr/bin/python
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# 4.2
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import scipy as sp
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from pylab import *
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a = 0.9
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n = sp.arange(-10,21)
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y = -(a**n*(n<=-1))
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stem(n,y,markerfmt='.')
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xlabel('Time(n)')
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ylabel(r'$-(0.9)^n1(-n-1)$')
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show()