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(gmp.info)Rational Internals

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Rational Internals
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   `mpq_t' variables represent rationals using an `mpz_t' numerator and
denominator (Note: Integer Internals).

   The canonical form adopted is denominator positive (and non-zero),
no common factors between numerator and denominator, and zero uniquely
represented as 0/1.

   It's believed that casting out common factors at each stage of a
calculation is best in general.  A GCD is an O(N^2) operation so it's
better to do a few small ones immediately than to delay and have to do
a big one later.  Knowing the numerator and denominator have no common
factors can be used for example in `mpq_mul' to make only two cross
GCDs necessary, not four.

   This general approach to common factors is badly sub-optimal in the
presence of simple factorizations or little prospect for cancellation,
but GMP has no way to know when this will occur.  As per Note:
Efficiency, that's left to applications.  The `mpq_t' framework might
still suit, with `mpq_numref' and `mpq_denref' for direct access to the
numerator and denominator, or of course `mpz_t' variables can be used
directly.


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