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In the documentation for Erfc (under "Possible Issues"), the following command returns a number that is extremely close to 2: enter image description here

However, when I run this same command in a fresh kernel, I get:

enter image description here

What's going on here? Wrapping the command in N[#, 1000] doesn't seem to help. I'm using Mathematica 8.0.1 on OS X.

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Wrapping the command in N will help when you remove the decimal point after the $30$. Ask for $407$ digits. –  whuber Mar 5 '13 at 4:20
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up vote 3 down vote accepted

I think there is a typo in the documentation. I think they meant to write something like

Erfc[-30.`20 + 10^-1 I]

which gives

1.99999999999999999999999999999999999999999999999999999999999999999999\ 9999999999999999999999999999999999999999999999999999999999999999999999\ 9999999999999999999999999999999999999999999999999999999999999999999999\ 9999999999999999999999999999999999999999999999999999999999999999999999\ 9999999999999999999999999999999999999999999999999999999999999999999999\ 9999999999999999999999999999999999999999999975103622845528878 + 7.155170950793436*10^-394 I

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There does appear to be some kind of a typo, but it is curious that the given answer in the docs looks like it should correspond to somewhere between $16$ and $17$ decimal places--almost exactly the native precision. It seems there might be something else going on here. –  whuber Mar 5 '13 at 4:52
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