# Permutation group elements¶

AUTHORS:

• David Joyner (2006-02)
• David Joyner (2006-03): word problem method and reorganization
• Robert Bradshaw (2007-11): convert to Cython
• Sebastian Oehms (2018-11): Added gap() as synonym to _gap_() (compatibility to libgap framework, see trac ticket #26750)
• Sebastian Oehms (2019-02): Implemented gap() properly (trac ticket #27234)

There are several ways to define a permutation group element:

• Define a permutation group $$G$$, then use G.gens() and multiplication * to construct elements.
• Define a permutation group $$G$$, then use, e.g., G([(1,2),(3,4,5)]) to construct an element of the group. You could also use G('(1,2)(3,4,5)')
• Use, e.g., PermutationGroupElement([(1,2),(3,4,5)]) or PermutationGroupElement('(1,2)(3,4,5)') to make a permutation group element with parent $$S_5$$.

EXAMPLES:

We illustrate construction of permutation using several different methods.

First we construct elements by multiplying together generators for a group:

sage: G = PermutationGroup(['(1,2)(3,4)', '(3,4,5,6)'], canonicalize=False)
sage: s = G.gens()
sage: s[0]
(1,2)(3,4)
sage: s[1]
(3,4,5,6)
sage: s[0]*s[1]
(1,2)(3,5,6)
sage: (s[0]*s[1]).parent()
Permutation Group with generators [(1,2)(3,4), (3,4,5,6)]


Next we illustrate creation of a permutation using coercion into an already-created group:

sage: g = G([(1,2),(3,5,6)])
sage: g
(1,2)(3,5,6)
sage: g.parent()
Permutation Group with generators [(1,2)(3,4), (3,4,5,6)]
sage: g == s[0]*s[1]
True


We can also use a string or one-line notation to specify the permutation:

sage: h = G('(1,2)(3,5,6)')
sage: i = G([2,1,5,4,6,3])
sage: g == h == i
True


The Rubik’s cube group:

sage: f = [(17,19,24,22),(18,21,23,20),( 6,25,43,16),( 7,28,42,13),( 8,30,41,11)]
sage: b = [(33,35,40,38),(34,37,39,36),( 3, 9,46,32),( 2,12,47,29),( 1,14,48,27)]
sage: l = [( 9,11,16,14),(10,13,15,12),( 1,17,41,40),( 4,20,44,37),( 6,22,46,35)]
sage: r = [(25,27,32,30),(26,29,31,28),( 3,38,43,19),( 5,36,45,21),( 8,33,48,24)]
sage: u = [( 1, 3, 8, 6),( 2, 5, 7, 4),( 9,33,25,17),(10,34,26,18),(11,35,27,19)]
sage: d = [(41,43,48,46),(42,45,47,44),(14,22,30,38),(15,23,31,39),(16,24,32,40)]
sage: cube = PermutationGroup([f, b, l, r, u, d])
sage: F, B, L, R, U, D = cube.gens()
sage: cube.order()
43252003274489856000
sage: F.order()
4


We create element of a permutation group of large degree:

sage: G = SymmetricGroup(30)
sage: s = G(srange(30,0,-1)); s
(1,30)(2,29)(3,28)(4,27)(5,26)(6,25)(7,24)(8,23)(9,22)(10,21)(11,20)(12,19)(13,18)(14,17)(15,16)

class sage.groups.perm_gps.permgroup_element.PermutationGroupElement

An element of a permutation group.

EXAMPLES:

sage: G = PermutationGroup(['(1,2,3)(4,5)'])
sage: G
Permutation Group with generators [(1,2,3)(4,5)]
sage: g = G.random_element()
sage: g in G
True
sage: g = G.gen(0); g
(1,2,3)(4,5)
sage: print(g)
(1,2,3)(4,5)
sage: g*g
(1,3,2)
sage: g**(-1)
(1,3,2)(4,5)
sage: g**2
(1,3,2)
sage: G = PermutationGroup([(1,2,3)])
sage: g = G.gen(0); g
(1,2,3)
sage: g.order()
3


This example illustrates how permutations act on multivariate polynomials.

sage: R = PolynomialRing(RationalField(), 5, ["x","y","z","u","v"])
sage: x, y, z, u, v = R.gens()
sage: f = x**2 - y**2 + 3*z**2
sage: G = PermutationGroup(['(1,2,3)(4,5)', '(1,2,3,4,5)'])
sage: sigma = G.gen(0)
sage: f * sigma
3*x^2 + y^2 - z^2

cycle_string(singletons=False)
Return string representation of this permutation.

EXAMPLES:

sage: g = PermutationGroupElement([(1,2,3),(4,5)])
sage: g.cycle_string()
'(1,2,3)(4,5)'

sage: g = PermutationGroupElement([3,2,1])
sage: g.cycle_string(singletons=True)
'(1,3)(2)'

cycle_tuples(singletons=False)

Return self as a list of disjoint cycles, represented as tuples rather than permutation group elements.

INPUT:

• singletons - boolean (default: False) whether or not consider the cycle that correspond to fixed point

EXAMPLES:

sage: p = PermutationGroupElement('(2,6)(4,5,1)')
sage: p.cycle_tuples()
[(1, 4, 5), (2, 6)]
sage: p.cycle_tuples(singletons=True)
[(1, 4, 5), (2, 6), (3,)]


EXAMPLES:

sage: S = SymmetricGroup(4)
sage: S.gen(0).cycle_tuples()
[(1, 2, 3, 4)]

sage: S = SymmetricGroup(['a','b','c','d'])
sage: S.gen(0).cycle_tuples()
[('a', 'b', 'c', 'd')]
sage: S([('a', 'b'), ('c', 'd')]).cycle_tuples()
[('a', 'b'), ('c', 'd')]

cycle_type(singletons=True, as_list=False)

Return the partition that gives the cycle type of g as an element of self.

INPUT:

• g – an element of the permutation group self.parent()
• singletonsTrue or False depending on whether on or not trivial cycles should be counted (default: True)
• as_listTrue or False depending on whether the cycle type should be returned as a list or as a Partition (default: False)

OUTPUT:

A Partition, or list if is_list is True, giving the cycle type of g

If speed is a concern then as_list=True should be used.

EXAMPLES:

sage: G = DihedralGroup(3)
sage: [g.cycle_type() for g in G]
[[1, 1, 1], [3], [3], [2, 1], [2, 1], [2, 1]]
sage: PermutationGroupElement('(1,2,3)(4,5)(6,7,8)').cycle_type()
[3, 3, 2]
sage: G = SymmetricGroup(3); G('(1,2)').cycle_type()
[2, 1]
sage: G = SymmetricGroup(4); G('(1,2)').cycle_type()
[2, 1, 1]
sage: G = SymmetricGroup(4); G('(1,2)').cycle_type(singletons=False)
[2]
sage: G = SymmetricGroup(4); G('(1,2)').cycle_type(as_list=False)
[2, 1, 1]

cycles()

Return self as a list of disjoint cycles.

EXAMPLES:

sage: G = PermutationGroup(['(1,2,3)(4,5,6,7)'])
sage: g = G.0
sage: g.cycles()
[(1,2,3), (4,5,6,7)]
sage: a, b = g.cycles()
sage: a(1), b(1)
(2, 1)

dict()

Returns a dictionary associating each element of the domain with its image.

EXAMPLES:

sage: G = SymmetricGroup(4)
sage: g = G((1,2,3,4)); g
(1,2,3,4)
sage: v = g.dict(); v
{1: 2, 2: 3, 3: 4, 4: 1}
sage: type(v[1])
<... 'int'>
sage: x = G([2,1]); x
(1,2)
sage: x.dict()
{1: 2, 2: 1, 3: 3, 4: 4}

domain()

Returns the domain of self.

EXAMPLES:

sage: G = SymmetricGroup(4)
sage: x = G([2,1,4,3]); x
(1,2)(3,4)
sage: v = x.domain(); v
[2, 1, 4, 3]
sage: type(v[0])
<... 'int'>
sage: x = G([2,1]); x
(1,2)
sage: x.domain()
[2, 1, 3, 4]

gap()

Returns self as a libgap element

EXAMPLES:

sage: P = PGU(8,2)
sage: p, q = P.gens()
sage: p_libgap  = p.gap()
sage: p_pexpect = gap(p)
sage: p_libgap == p_pexpect
True
sage: type(p_libgap) == type(p_pexpect)
False

has_descent(i, side='right', positive=False)

INPUT:

• i: an element of the index set
• side: “left” or “right” (default: “right”)
• positive: a boolean (default: False)

Returns whether self has a left (resp. right) descent at position i. If positive is True, then test for a non descent instead.

Beware that, since permutations are acting on the right, the meaning of descents is the reverse of the usual convention. Hence, self has a left descent at position i if self(i) > self(i+1).

EXAMPLES:

sage: S = SymmetricGroup([1,2,3])
sage: S.one().has_descent(1)
False
sage: S.one().has_descent(2)
False
sage: s = S.simple_reflections()
sage: x = s[1]*s[2]
sage: x.has_descent(1, side = "right")
False
sage: x.has_descent(2, side = "right")
True
sage: x.has_descent(1, side = "left")
True
sage: x.has_descent(2, side = "left")
False
sage: S._test_has_descent()


The symmetric group acting on a set not of the form $$(1,\dots,n)$$ is also supported:

sage: S = SymmetricGroup([2,4,1])
sage: s = S.simple_reflections()
sage: x = s[2]*s[4]
sage: x.has_descent(4)
True
sage: S._test_has_descent()

inverse()

Returns the inverse permutation.

OUTPUT:

For an element of a permutation group, this method returns the inverse element, which is both the inverse function and the inverse as an element of a group.

EXAMPLES:

sage: s = PermutationGroupElement("(1,2,3)(4,5)")
sage: s.inverse()
(1,3,2)(4,5)

sage: A = AlternatingGroup(4)
sage: t = A("(1,2,3)")
sage: t.inverse()
(1,3,2)


There are several ways (syntactically) to get an inverse of a permutation group element.

sage: s = PermutationGroupElement("(1,2,3,4)(6,7,8)")
sage: s.inverse() == s^-1
True
sage: s.inverse() == ~s
True

matrix()

Returns deg x deg permutation matrix associated to the permutation self

EXAMPLES:

sage: G = PermutationGroup(['(1,2,3)(4,5)'])
sage: g = G.gen(0)
sage: g.matrix()
[0 1 0 0 0]
[0 0 1 0 0]
[1 0 0 0 0]
[0 0 0 0 1]
[0 0 0 1 0]

multiplicative_order()

Return the order of this group element, which is the smallest positive integer $$n$$ for which $$g^n = 1$$.

EXAMPLES:

sage: s = PermutationGroupElement('(1,2)(3,5,6)')
sage: s.multiplicative_order()
6


order is just an alias for multiplicative_order:

sage: s.order()
6

orbit(n, sorted=True)

Returns the orbit of the integer $$n$$ under this group element, as a sorted list.

EXAMPLES:

sage: G = PermutationGroup(['(1,2,3)(4,5)'])
sage: g = G.gen(0)
sage: g.orbit(4)
[4, 5]
sage: g.orbit(3)
[1, 2, 3]
sage: g.orbit(10)
[10]

sage: s = SymmetricGroup(['a', 'b']).gen(0); s
('a','b')
sage: s.orbit('a')
['a', 'b']

sign()

Returns the sign of self, which is $$(-1)^{s}$$, where $$s$$ is the number of swaps.

EXAMPLES:

sage: s = PermutationGroupElement('(1,2)(3,5,6)')
sage: s.sign()
-1


ALGORITHM: Only even cycles contribute to the sign, thus

$sign(sigma) = (-1)^{\sum_c len(c)-1}$

where the sum is over cycles in self.

tuple()

Return tuple of images of the domain under self.

EXAMPLES:

sage: G = SymmetricGroup(5)
sage: s = G([2,1,5,3,4])
sage: s.tuple()
(2, 1, 5, 3, 4)

sage: S = SymmetricGroup(['a', 'b'])
sage: S.gen().tuple()
('b', 'a')

word_problem(words, display=True)

G and H are permutation groups, g in G, H is a subgroup of G generated by a list (words) of elements of G. If g is in H, return the expression for g as a word in the elements of (words).

This function does not solve the word problem in Sage. Rather it pushes it over to GAP, which has optimized algorithms for the word problem. Essentially, this function is a wrapper for the GAP functions “EpimorphismFromFreeGroup” and “PreImagesRepresentative”.

EXAMPLES:

sage: G = PermutationGroup([[(1,2,3),(4,5)],[(3,4)]], canonicalize=False)
sage: g1, g2 = G.gens()
sage: h = g1^2*g2*g1
sage: h.word_problem([g1,g2], False)
('x1^2*x2^-1*x1', '(1,2,3)(4,5)^2*(3,4)^-1*(1,2,3)(4,5)')
sage: h.word_problem([g1,g2])
x1^2*x2^-1*x1
[['(1,2,3)(4,5)', 2], ['(3,4)', -1], ['(1,2,3)(4,5)', 1]]
('x1^2*x2^-1*x1', '(1,2,3)(4,5)^2*(3,4)^-1*(1,2,3)(4,5)')

class sage.groups.perm_gps.permgroup_element.SymmetricGroupElement

An element of the symmetric group.

absolute_length()

Return the absolute length of self.

The absolute length is the size minus the number of its disjoint cycles. Alternatively, it is the length of the shortest expression of the element as a product of reflections.

absolute_le()

EXAMPLES:

sage: S = SymmetricGroup(3)
sage: [x.absolute_length() for x in S]
[0, 2, 2, 1, 1, 1]

has_left_descent(i)

Return whether $$i$$ is a left descent of self.

EXAMPLES:

sage: W = SymmetricGroup(4)
sage: w = W.from_reduced_word([1,3,2,1])
sage: [i for i in W.index_set() if w.has_left_descent(i)]
[1, 3]

sage.groups.perm_gps.permgroup_element.is_PermutationGroupElement(x)

Returns True if x is a PermutationGroupElement.

EXAMPLES:

sage: p = PermutationGroupElement([(1,2),(3,4,5)])
sage: from sage.groups.perm_gps.permgroup_element import is_PermutationGroupElement
sage: is_PermutationGroupElement(p)
True

sage.groups.perm_gps.permgroup_element.make_permgroup_element(G, x)

Returns a PermutationGroupElement given the permutation group G and the permutation x in list notation.

This is function is used when unpickling old (pre-domain) versions of permutation groups and their elements. This now does a bit of processing and calls make_permgroup_element_v2() which is used in unpickling the current PermutationGroupElements.

EXAMPLES:

sage: from sage.groups.perm_gps.permgroup_element import make_permgroup_element
sage: S = SymmetricGroup(3)
sage: make_permgroup_element(S, [1,3,2])
(2,3)

sage.groups.perm_gps.permgroup_element.make_permgroup_element_v2(G, x, domain)

Returns a PermutationGroupElement given the permutation group G, the permutation x in list notation, and the domain domain of the permutation group.

This is function is used when unpickling permutation groups and their elements.

EXAMPLES:

sage: from sage.groups.perm_gps.permgroup_element import make_permgroup_element_v2
sage: S = SymmetricGroup(3)
sage: make_permgroup_element_v2(S, [1,3,2], S.domain())
(2,3)

sage.groups.perm_gps.permgroup_element.standardize_generator(g, convert_dict=None)

Standardizes the input for permutation group elements to a list of tuples. This was factored out of the PermutationGroupElement.__init__ since PermutationGroup_generic.__init__ needs to do the same computation in order to compute the domain of a group when it’s not explicitly specified.

INPUT:

• g - a list, tuple, string, GapElement, PermutationGroupElement, Permutation
• convert_dict - (optional) a dictionary used to convert the points to a number compatible with GAP.

OUTPUT:

The permutation in as a list of cycles.

EXAMPLES:

sage: from sage.groups.perm_gps.permgroup_element import standardize_generator
sage: standardize_generator('(1,2)')
[(1, 2)]

sage: p = PermutationGroupElement([(1,2)])
sage: standardize_generator(p)
[(1, 2)]
sage: standardize_generator(p._gap_())
[(1, 2)]
sage: standardize_generator((1,2))
[(1, 2)]
sage: standardize_generator([(1,2)])
[(1, 2)]
sage: standardize_generator(Permutation([2,1,3]))
[(1, 2), (3,)]

sage: d = {'a': 1, 'b': 2}
sage: p = SymmetricGroup(['a', 'b']).gen(0); p
('a','b')
sage: standardize_generator(p, convert_dict=d)
[(1, 2)]
sage: standardize_generator(p._gap_(), convert_dict=d)
[(1, 2)]
sage: standardize_generator(('a','b'), convert_dict=d)
[(1, 2)]
sage: standardize_generator([('a','b')], convert_dict=d)
[(1, 2)]

sage.groups.perm_gps.permgroup_element.string_to_tuples(g)

EXAMPLES:

sage: from sage.groups.perm_gps.permgroup_element import string_to_tuples
sage: string_to_tuples('(1,2,3)')
[(1, 2, 3)]
sage: string_to_tuples('(1,2,3)(4,5)')
[(1, 2, 3), (4, 5)]
sage: string_to_tuples(' (1,2, 3) (4,5)')
[(1, 2, 3), (4, 5)]
sage: string_to_tuples('(1,2)(3)')
[(1, 2), (3,)]