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### `igcd`,`igcdcntl`

`igcd(i1,i2)`
  :: The integer greatest common divisor of i1 and i2.
`igcdcntl([i])`
  :: Selects an algorithm for integer GCD.

return
  integer
i1,i2,i
  integer

- Function `igcd()` returns the integer greatest common divisor of the given two integers.
- An error will result if the argument is not an integer; the result is not valid even if one is returned.
- Use `gcd()`, `gcdz()` for polynomial GCD.
- Various method of integer GCD computation are implemented and they can be selected by `igcdcntl`.
  `0`
    Euclid algorithm (default)
  `1`
    binary GCD
  `2`
    bmod GCD
  `3`
    accelerated integer GCD
  `2`, `3` are due to `[Weber]`. In most cases `3` is the fastest, but there are exceptions.

    [0] A=lrandom(10^4)$
    [1] B=lrandom(10^4)$
    [2] C=lrandom(10^4)$
    [3] D=A*C$
    [4] E=A*B$
    [5] cputime(1)$
    [6] igcd(D,E)$
    0.6sec + gc : 1.93sec(2.531sec)
    [7] igcdcntl(1)$
    [8] igcd(D,E)$
    0.27sec(0.2635sec)
    [9] igcdcntl(2)$
    [10] igcd(D,E)$
    0.19sec(0.1928sec)
    [11] igcdcntl(3)$
    [12] igcd(D,E)$
    0.08sec(0.08023sec)

`References`
  section `gcd`, `gcdz`.

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