galois.primitive_root

galois.primitive_root(n: int, start: int = 1, stop: int | None = None, method: 'min' | 'max' | 'random' = 'min') → int

Finds a primitive root modulo \(n\) in the range [start, stop).

Parameters
n: int

A positive integer.

start: int = 1

Starting value (inclusive) in the search for a primitive root.

stop: int | None = None

Stopping value (exclusive) in the search for a primitive root. The default is None which corresponds to \(n\).

method: 'min' | 'max' | 'random' = 'min'

The search method for finding the primitive root.

Returns

A primitive root modulo \(n\) in the specified range.

Raises

RuntimeError – If no primitive roots exist in the specified range.

See also

primitive_roots, is_primitive_root, is_cyclic, totatives, euler_phi, carmichael_lambda

Notes

The integer \(g\) is a primitive root modulo \(n\) if the totatives of \(n\) can be generated by the powers of \(g\). The totatives of \(n\) are the positive integers in \([1, n)\) that are coprime with \(n\).

Alternatively said, \(g\) is a primitive root modulo \(n\) if and only if \(g\) is a generator of the multiplicative group of integers modulo \(n\) \((\mathbb{Z}/n\mathbb{Z}){^\times} = \{1, g, g^2, \dots, g^{\phi(n)-1}\}\), where \(\phi(n)\) is order of the group.

If \((\mathbb{Z}/n\mathbb{Z}){^\times}\) is cyclic, the number of primitive roots modulo \(n\) is given by \(\phi(\phi(n))\).

References

Examples

The elements of \((\mathbb{Z}/14\mathbb{Z}){^\times} = \{1, 3, 5, 9, 11, 13\}\) are the totatives of \(14\).

In [1]: n = 14

In [2]: Znx = galois.totatives(n); Znx
Out[2]: [1, 3, 5, 9, 11, 13]

The Euler totient \(\phi(n)\) function counts the totatives of \(n\), which is equivalent to the order of \((\mathbb{Z}/n\mathbb{Z}){^\times}\).

In [3]: phi = galois.euler_phi(n); phi
Out[3]: 6

In [4]: len(Znx) == phi
Out[4]: True

Since \(14\) is of the form \(2p^k\), the multiplicative group \((\mathbb{Z}/14\mathbb{Z}){^\times}\) is cyclic, meaning there exists at least one element that generates the group by its powers.

In [5]: galois.is_cyclic(n)
Out[5]: True

Find the smallest primitive root modulo \(14\). Observe that the powers of \(g\) uniquely represent each element in \((\mathbb{Z}/14\mathbb{Z}){^\times}\).

In [6]: g = galois.primitive_root(n); g
Out[6]: 3

In [7]: [pow(g, i, n) for i in range(0, phi)]
Out[7]: [1, 3, 9, 13, 11, 5]

Find the largest primitive root modulo \(14\). Observe that the powers of \(g\) also uniquely represent each element in \((\mathbb{Z}/14\mathbb{Z}){^\times}\), although in a different order.

In [8]: g = galois.primitive_root(n, method="max"); g
Out[8]: 5

In [9]: [pow(g, i, n) for i in range(0, phi)]
Out[9]: [1, 5, 11, 13, 9, 3]

A non-cyclic group is \((\mathbb{Z}/15\mathbb{Z}){^\times} = \{1, 2, 4, 7, 8, 11, 13, 14\}\).

In [10]: n = 15

In [11]: Znx = galois.totatives(n); Znx
Out[11]: [1, 2, 4, 7, 8, 11, 13, 14]

In [12]: phi = galois.euler_phi(n); phi
Out[12]: 8

Since \(15\) is not of the form \(2\), \(4\), \(p^k\), or \(2p^k\), the multiplicative group \((\mathbb{Z}/15\mathbb{Z}){^\times}\) is not cyclic, meaning no elements exist whose powers generate the group.

In [13]: galois.is_cyclic(n)
Out[13]: False

Below, every element is tested to see if it spans the group.

In [14]: for a in Znx:
   ....:     span = set([pow(a, i, n) for i in range(0, phi)])
   ....:     primitive_root = span == set(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

The Carmichael \(\lambda(n)\) function finds the maximum multiplicative order of any element, which is \(4\) and not \(8\).

In [15]: galois.carmichael_lambda(n)
Out[15]: 4

Observe that no primitive roots modulo \(15\) exist and a RuntimeError is raised.

In [16]: galois.primitive_root(n)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
~/repos/galois/galois/_modular.py in primitive_root(n, start, stop, method)
    575         if method == "min":
--> 576             return next(primitive_roots(n, start, stop=stop))
    577         elif method == "max":

StopIteration: 

The above exception was the direct cause of the following exception:

RuntimeError                              Traceback (most recent call last)
<ipython-input-16-bb1af462e9d6> in <module>
----> 1 galois.primitive_root(n)

~/repos/galois/galois/_modular.py in primitive_root(n, start, stop, method)
    580             return _primitive_root_random_search(n, start, stop)
    581     except StopIteration as e:
--> 582         raise RuntimeError(f"No primitive roots modulo {n} exist in the range [{start}, {stop}).") from e
    583 
    584 

RuntimeError: No primitive roots modulo 15 exist in the range [1, 15).

The algorithm is also efficient for very large \(n\).

In [17]: n = 1000000000000000035000061

In [18]: phi = galois.euler_phi(n); phi
Out[18]: 1000000000000000035000060

Find the smallest, the largest, and a random primitive root modulo \(n\).

In [19]: galois.primitive_root(n)
Out[19]: 7

In [20]: galois.primitive_root(n, method="max")
Out[20]: 1000000000000000035000054

In [21]: galois.primitive_root(n, method="random")
Out[21]: 465069391317962060497599
Last update: Apr 21, 2022