Input resistance for AC input

I read on a book that when a circuit has an AC voltage source as the input then it's input resistance is
Ri= ui/ii
where ui is the instantaneous input voltage and ii the
instantaneous input current.
Isn't this false?
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It's not a definition that I've seen. It doesn't strike me as a good definition in general because the math would get a little dicey at the zero crossings. Otherwise I guess it would be basically correct for a resistive network, even if resistances changed with time.
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Yes. Its wrong. Input resistance of an ideal voltage source is Zero. pls click the following link for more iformation http://en.wikipedia.org/wiki/Voltage_source
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remove the X to answer ----------------------------
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