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The value isn't critical. I'd also recommend adding a 100nF ceramic capacitor to the output as it has better high frequency characteristics. The addition of capacitors is known as bypassing or decoupling and it touched on in the LM7805 data sheet.
http://www.ti.com/lit/ds/symlink/lm340-n.pdf

A more detailed explanation can be found on the LM317 data sheet.
http://www.ti.com/lit/ds/symlink/lm117.pdf

Here are a couple of links with more information:
http://www.ti.com/lit/an/snva020a/snva020a.pdf
http://www.ti.com/lit/an/snva558/snva558.pdf

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can you tell me more about this statement about these values

"The value isn't critical but if you make it too small you risk it being inadequate to damp oscillation. If you make it too big, at least with a single capacitor, you run the risk of the high frequency impedance of the capacitor being too high.  "

damp oscillation and high frequency impedance  ?

thanks

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All that means is that if the capacitor is too small, it won't be large enough to damp the oscillations but if it's a single, large value capacitor it won't be good either. Higher value capacitors tend to have poor high frequency characteristics and become inductive past the resonant frequency, the higher the value, the lower the resonant frequency.

Rather than using a single, high value capacitor, it's better to use a large capacitor and lower value capacitor in parallel with one another. The type of capacitor is also important; for example tantalum is better than electrolytic. I'd recommend a 1uF tantalum in parallel with a 100nF and 10nF ceramic capacitor on both the input and output. If you don't have a 1uf  tantalum, go with 22uF electrolytic.

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