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The Geometric Series formulas below still hold for <math> \alpha\ </math>'s containing complex exponentials. | The Geometric Series formulas below still hold for <math> \alpha\ </math>'s containing complex exponentials. | ||
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In this case, <math> \alpha=\frac{1}{2}e^{-j\omega} </math> in the above Geometric Series formula. | In this case, <math> \alpha=\frac{1}{2}e^{-j\omega} </math> in the above Geometric Series formula. | ||
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| + | [[More_on_geometric_series|More on geometric series]] | ||
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Revision as of 09:26, 7 September 2011
The Geometric Series formulas below still hold for $ \alpha\ $'s containing complex exponentials.
For k from 0 to n, where $ \alpha \ne 1 $:
- $ \sum_{k=0}^{n} \alpha^k = \frac{1-\alpha^{n+1}}{1-\alpha} $
- (else, = n + 1)
For k from 0 to $ \infty\ $, where $ \alpha < 1\ $:
- $ \sum_{k=0}^\infty \alpha^k = \frac{1}{1-\alpha} $
- (else it diverges)
Example: We want to evaluate the following:
- $ \sum_{k=0}^\infty (\frac{1}{2})^k e^{-j \omega k}= \sum_{k=0}^\infty (\frac{1}{2}e^{-j\omega})^k = \frac{1}{1-\frac{1}{2}e^{-j\omega}} $
In this case, $ \alpha=\frac{1}{2}e^{-j\omega} $ in the above Geometric Series formula.
