I need to find the $\mathcal Z$-transform of $x(n)=a^{-n} u(n)$. Assume, $a$ is a positive constant , but the power of $a$ is negative.

Looking at the transform table, I found that $\mathcal Z$-transform for $x(n)=a^{-n} u(n)$ is not there, so I tried to found it by on my own, but I am not sure if my answer is right, please can anyone check it and correct me, in case?


Doing some calculations is appreciated. However, it is preferred that you write them in the question because a picture can vanish more easily. Do not hesitate to edit the math.

To help you with an hint in checking your answer, remember that:

$$ a^{-n} = \left(\frac{1}{a}\right)^n$$

a standard trick to play with known tables.


No you have done a small mistake how a^-n became a^n? So it is like this the ROC is right sidedthe procedure is like this

  • 3
    $\begingroup$ Please, use the formatting options provided by the site to write your answer instead of posting images. $\endgroup$ – Tendero Mar 19 '17 at 14:35
  • $\begingroup$ I don't know how to do such things $\endgroup$ – Ch.Siva Ram Kishore Mar 19 '17 at 14:37
  • 1
    $\begingroup$ You can check out how to improve your formatting here. $\endgroup$ – Tendero Mar 19 '17 at 15:02
  • $\begingroup$ Moni alfarsia if you put your question as answered then it is better $\endgroup$ – Ch.Siva Ram Kishore Mar 19 '17 at 15:20
  • $\begingroup$ Ok , Iam sorry but Its my first time that I have post in this website , In second time make sure that I will not post a picture . $\endgroup$ – moni alfarsia Mar 19 '17 at 15:49

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