--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Z-trans/aconv2.py	Fri May 27 14:24:59 2011 +0530
@@ -0,0 +1,12 @@
+#!/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()