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B. Floating Point Arithmetic: Issues and Limitations http://www.vithon.org/tutorial/2.5/node16.html

So the computer never ``sees'' 1/10: what it sees is the exact fraction given
above, the best 754 double approximation it can get:

>>> .1 * 2**56
7205759403792794.0

If we multiply that fraction by 10**30, we can see the (truncated) value of its
30 most significant decimal digits:

>>> 7205759403792794 * 10**30 / 2**56
100000000000000005551115123125L

meaning that the exact number stored in the computer is approximately
equal to the decimal value 0.100000000000000005551115123125. Rounding
that to 17 significant digits gives the 0.10000000000000001 that Python
displays (well, will display on any 754-conforming platform that does
best-possible input and output conversions in its C library -- yours may not!).

Release 2.5, documentation updated on 19th September, 2006.
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