- Added some special cases in Number `cyclotomic(n,x)`.
- Added some special cases in Number `cyclotomic(n,x)`.

	cyclotomic(0, x) = 1
	cyclotomic(1, x) = x-1
	cyclotomic(2, x) = x+1

Also:

	cyclotomic(n, 1) is the greatest common divisor of the prime factors of n.

See also: https://oeis.org/A020500

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