6.28.12 Cyclotomic polynomial: cyclotomic
For a positive integer n, cyclotomic polynomial of index n is
the monic polynomial whose roots are exactly the primitive nth roots
of unity (an nth root of unity is primitive if the set of
its powers is the set of all the nth roots of unity). Note that
this will divide xn−1, whose roots are all the nth roots of
unity.
The cyclotomic command computes cyclotomic polynomials.
-
cyclotomic takes one argument:
n, an integer.
- cyclotomic(n) returns the list of the coefficients
of the cyclotomic polynomial of index n.
Examples.
-
Let n=4; the fourth roots of unity are: {1,i,−1,−i} and
the primitive roots are: {i,−i}. Hence, the cyclotomic polynomial
of index 4 is (x−i)(x+i)=x2+1.
Input (for verification):
cyclotomic(4)
Output:
- Input:
cyclotomic(5)
Output:
Hence, the cyclotomic polynomial of index 5 is x4+x3+x2+x+1,
which divides x5−1 since (x−1)*(x4+x3+x2+x+1)=x5−1.
- Input:
cyclotomic(10)
Output:
Hence, the cyclotomic polynomial of index 10 is x4−x3+x2−x+1 and
(x5−1)*(x+1)*(x4−x3+x2−x+1)=x10−1
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- Input:
cyclotomic(20)
Output:
⎡
⎣ | 1,0,−1,0,1,0,−1,0,1 | ⎤
⎦ |
Hence, the cyclotomic polynomial of index 20 is x8−x6+x4−x2+1 and
(x10−1)(x2+1)*(x8−x6+x4−x2+1)=x20−1
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