galois.is_cyclic¶
-
galois.
is_cyclic
(n)[source]¶ Determines whether the multiplicative group \(\mathbb{Z}{_n^\times}\) is cyclic.
The multiplicative group \(\mathbb{Z}{_n^\times}\) is the set of positive integers \(1 \le a < n\) that are coprime with \(n\). \(\mathbb{Z}{_n^\times}\) being cyclic means that some primitive root (or generator) \(g\) can generate the group \(\mathbb{Z}{_n^\times} = \{g, g^2, \dots, g^k\}\), where \(k\) is order of the group. The order of the group is defined by Euler’s totient function, \(\phi(n) = k\). If \(\mathbb{Z}{_n^\times}\) is cyclic, the number of primitive roots is found by \(\phi(k)\) or \(\phi(\phi(n))\).
\(\mathbb{Z}{_n^\times}\) is cyclic if and only if \(n\) is \(2\), \(4\), \(p^k\), or \(2p^k\), where \(p\) is an odd prime and \(k\) is a positive integer.
- Parameters
n (int) – A positive integer.
- Returns
True
if the multiplicative group \(\mathbb{Z}{_n^\times}\) is cyclic.- Return type
References
Examples
The elements of \(\mathbb{Z}{_n^\times}\) are the positive integers less than \(n\) that are coprime with \(n\). For example when \(n = 14\), then \(\mathbb{Z}{_{14}^\times} = \{1, 3, 5, 9, 11, 13\}\).
# n is of type 2*p^k, which is cyclic In [1]: n = 14 In [2]: galois.is_cyclic(n) Out[2]: True # The congruence class coprime with n In [3]: Znx = set([a for a in range(1, n) if math.gcd(n, a) == 1]); Znx Out[3]: {1, 3, 5, 9, 11, 13} # Euler's totient function counts the "totatives", positive integers coprime with n In [4]: phi = galois.euler_totient(n); phi Out[4]: 6 In [5]: len(Znx) == phi Out[5]: True # The primitive roots are the elements in Znx that multiplicatively generate the group In [6]: for a in Znx: ...: span = set([pow(a, i, n) for i in range(1, phi + 1)]) ...: primitive_root = span == Znx ...: print("Element: {:2d}, Span: {:<20}, Primitive root: {}".format(a, str(span), primitive_root)) ...: Element: 1, Span: {1} , Primitive root: False Element: 3, Span: {1, 3, 5, 9, 11, 13}, Primitive root: True Element: 5, Span: {1, 3, 5, 9, 11, 13}, Primitive root: True Element: 9, Span: {9, 11, 1} , Primitive root: False Element: 11, Span: {9, 11, 1} , Primitive root: False Element: 13, Span: {1, 13} , Primitive root: False In [7]: roots = galois.primitive_roots(n); roots Out[7]: [3, 5] # Euler's totient function phi(phi(n)) counts the primitive roots of n In [8]: len(roots) == galois.euler_totient(phi) Out[8]: True
A counterexample is \(n = 15 = 3*5\), which doesn’t fit the condition for cyclicness. \(\mathbb{Z}{_{15}^\times} = \{1, 2, 4, 7, 8, 11, 13, 14\}\).
# n is of type p1^k1 * p2^k2, which is not cyclic In [9]: n = 15 In [10]: galois.is_cyclic(n) Out[10]: False # The congruence class coprime with n In [11]: Znx = set([a for a in range(1, n) if math.gcd(n, a) == 1]); Znx Out[11]: {1, 2, 4, 7, 8, 11, 13, 14} # Euler's totient function counts the "totatives", positive integers coprime with n In [12]: phi = galois.euler_totient(n); phi Out[12]: 8 In [13]: len(Znx) == phi Out[13]: True # The primitive roots are the elements in Znx that multiplicatively generate the group In [14]: for a in Znx: ....: span = set([pow(a, i, n) for i in range(1, phi + 1)]) ....: primitive_root = span == Znx ....: print("Element: {:2d}, Span: {:<13}, Primitive root: {}".format(a, str(span), primitive_root)) ....: Element: 1, Span: {1} , Primitive root: False Element: 2, Span: {8, 1, 2, 4} , Primitive root: False Element: 4, Span: {1, 4} , Primitive root: False Element: 7, Span: {1, 4, 13, 7}, Primitive root: False Element: 8, Span: {8, 1, 2, 4} , Primitive root: False Element: 11, Span: {1, 11} , Primitive root: False Element: 13, Span: {1, 4, 13, 7}, Primitive root: False Element: 14, Span: {1, 14} , Primitive root: False In [15]: roots = galois.primitive_roots(n); roots Out[15]: [] # Note the max order of any element is 4, not 8, which is Carmichael's lambda function In [16]: galois.carmichael(n) Out[16]: 4