Basics

Elements of groups

Given a group G, it is always possible to have access to some particular elements.

GAPGroupType
GAPGroup <: AbstractAlgebra.Group

Each object of the abstract type GAPGroup stores a group object from the GAP system, and thus can delegate questions about this object to GAP.

For expert usage, you can extract the underlying GAP object via GapObj, i.e., if G is a GAPGroup, then GapObj(G) is the GapObj underlying G.

Concrete subtypes of GAPGroup are PermGroup, FPGroup, PcGroup, and MatrixGroup.

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BasicGAPGroupElemType
BasicGAPGroupElem{T<:GAPGroup} <: GAPGroupElem{T}

The type BasicGAPGroupElem gathers all types of group elements described only by an underlying GAP object.

If $x$ is an element of the group G of type T, then the type of $x$ is BasicGAPGroupElem{T}.

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elem_typeMethod
elem_type(::Type{T}) where T <: GAPGroup
elem_type(::T) where T <: GAPGroup

elem_type maps (the type of) a group to the type of its elements. For now, a group of type T has elements of type BasicGAPGroupElem{T}. So we provide it mostly for consistency with other parts of OSCAR. In the future, a more elaborate setup for group element types might also be needed.

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oneMethod
one(G::GAPGroup) -> elem_type(G)

Return the identity of the group G.

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oneMethod
one(x::GAPGroupElem{T}) -> GAPGroupElem{T}

Return the identity of the parent group of x.

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is_finiteorderMethod
is_finiteorder(g::GAPGroupElem) -> Bool

Return true if g has finite order, and false otherwise.

Examples

julia> is_finiteorder(gen(symmetric_group(5), 1))
true

julia> is_finiteorder(gen(free_group(2), 1))
false
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gensMethod
gens(G::Group)

Return a vector of generators of G. To get the i-th generator, use G[i] or gen(G,i) (see gen) instead of gens(G)[i], as that is more efficient.

Examples

julia> g = symmetric_group(5);  gens(g)
2-element Vector{PermGroupElem}:
 (1,2,3,4,5)
 (1,2)

julia> g[2]
(1,2)
Note

The output of gens(G) is not, in general, the minimal list of generators for G.

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has_gensMethod
has_gens(G::Group)

Return whether generators for the group G are known.

Examples

julia> F = free_group(2)
<free group on the generators [ f1, f2 ]>

julia> has_gens(F)
true

julia> H = derived_subgroup(F)[1]
Group(<free, no generators known>)

julia> has_gens(H)
false
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ngensMethod
ngens(G::GAPGroup) -> Int

Return the length of the vector gens(G).

WARNING:

this is NOT, in general, the minimum number of generators for G.

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genMethod
gen(G::GAPGroup, i::Int)

Return the i-th element of the vector gens(G). This is equivalent to G[i], and returns gens(G)[i] but may be more efficient than the latter.

An exception is thrown if i is larger than the length of gens(G).

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randMethod
rand(rng::Random.AbstractRNG = Random.GLOBAL_RNG, G::Group)

Return a random element of G, using the random number generator rng.

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rand_pseudoMethod
rand_pseudo(G::Group)

Return a pseudo random element of G. This works faster than rand, but the returned elements are not necessarily uniformly distributed.

It is sometimes necessary to work with finite groups that we cannot effectively enumerate, e.g. matrix groups over finite fields. We may not even know the size of these groups. Yet many algorithms need to sample elements from the group "as randomly as possible", whatever that means; but also they need this fast.

The function rand_pseudo returns elements that are cheap to compute and somehow random, but makes no guarantees about their distribution.

For finitely presented groups, it returns random words of bounded length.

For finite permutation and matrix groups, it uses a variant of the product replacement algorithm. For most inputs, the resulting stream of elements relatively quickly converges to a uniform distribution.

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It is also possible to obtain the generators of G by typing

f1,f2,f3 = gens(G)

This is equivalent to

f1=G[1]; f2=G[2]; f3=G[3];

For a group G that has been created as a subgroup of another group, generated by a list L of elements, gens(G) is equal to L.

Operations on group elements

OSCAR supports the following operations and functions on group elements.

  • *, multiplication between two elements in a group.
  • inv(x) and x^-1, the inverse of x.
  • x/y, the element x y^-1.
  • x^n, the n-th power of x; if n == 0, the identity of the group is returned; if n < 0, the -n-th power of the inverse of x is returned.
  • isone(x) returns whether x is the identity of the group.
  • conj(x,y) and x^y, the conjugate of x by y, i.e., the element y^-1 x y.
  • comm(x,y), the commutator of x and y, i.e., the element x^-1 y^-1 x y.
Note

In OSCAR, the expression x^y^z is equivalent to x^(y^z). In other words, conjugations are evaluated from the right to the left.

commMethod
comm(x::GAPGroupElem, y::GAPGroupElem)

Return the commutator of x and y, which is defined as x^-1*y^-1*x*y, and usually denoted as [x,y] in the literature.

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Properties of groups

isfiniteMethod
isfinite(G::GAPGroup) -> Bool

Return true if G is finite, and false otherwise.

Examples

julia> isfinite(symmetric_group(5))
true

julia> isfinite(free_group(2))
false
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is_abelianMethod
is_abelian(G::Group)

Return true if G is abelian (commutative), that is, $x*y = y*x$ holds for all elements $x, y$ in G.

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is_elementary_abelianFunction
is_elementary_abelian(G::Group)

Return true if G is a abelian (see is_abelian) and if there is a prime p such that the order of each element in G divides p.

Examples

julia> g = alternating_group(5);

julia> is_elementary_abelian(sylow_subgroup(g, 2)[1])
true

julia> g = alternating_group(6);

julia> is_elementary_abelian(sylow_subgroup(g, 2)[1])
false
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is_pgroupFunction
is_pgroup(G)

Return true if G is a $p$-group for some prime $p$, that is, if the order of every element in G is a power of $p$.

Note that a finite group is a $p$-group if and only if its order is a prime power. In particular, the trivial group is a $p$-group for every prime.

Examples

julia> is_pgroup(symmetric_group(1))
true

julia> is_pgroup(symmetric_group(2))
true

julia> is_pgroup(symmetric_group(3))
false
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is_pgroup_with_primeFunction
is_pgroup_with_prime(G)

Return (true, nothing) if G is the trivial group, (true, p) if the order of every element in G is a power of a prime p, and (false, nothing) otherwise.

For finite groups G, the first return value is true if and only if the order of G is a prime power.

Examples

julia> is_pgroup_with_prime(symmetric_group(1))
(true, nothing)

julia> is_pgroup_with_prime(symmetric_group(2))
(true, 2)

julia> is_pgroup_with_prime(symmetric_group(3))
(false, nothing)
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is_nilpotentFunction
is_nilpotent(G::GAPGroup)

Return whether G is nilpotent, i.e., whether the lower central series of G reaches the trivial subgroup in a finite number of steps.

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is_supersolvableFunction
is_supersolvable(G::GAPGroup)

Return whether G is supersolvable, i.e., G is finite and has a normal series with cyclic factors.

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is_solvableFunction
is_solvable(G::GAPGroup)

Return whether G is solvable, i.e., whether derived_series(G) reaches the trivial subgroup in a finite number of steps.

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is_perfectFunction
is_perfect(G::GAPGroup)

Return whether G is a perfect group, i.e., equal to its derived subgroup.

Examples

julia> is_perfect(special_linear_group(2, 5))
true

julia> is_perfect(symmetric_group(5))
false
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is_simpleMethod
is_simple(G::GAPGroup)

Return whether G is a simple group, i.e., G is not trivial and has no non-trivial normal subgroups.

Examples

julia> is_simple(alternating_group(5))
true

julia> is_simple(symmetric_group(5))
false
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is_almostsimpleFunction
is_almostsimple(G::GAPGroup)

Return whether G is an almost simple group, i.e., G is isomorphic to a group $H$ with the property $S \leq H \leq Aut(S)$, for some non-abelian simple group $S$.

Examples

julia> is_almostsimple(symmetric_group(5))
true

julia> is_almostsimple(special_linear_group(2, 5))
false
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is_quasisimpleFunction
is_quasisimple(G::GAPGroup)

Return whether G is a quasisimple group, i.e., G is perfect such that the factor group modulo its centre is a non-abelian simple group.

Examples

julia> is_quasisimple(special_linear_group(2, 5))
true

julia> is_quasisimple(symmetric_group(5))
false
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is_sporadic_simpleFunction
is_sporadic_simple(G::GAPGroup)

Return whether G is a sporadic simple group.

Examples

julia> is_sporadic_simple(mathieu_group(11))
true

julia> is_sporadic_simple(mathieu_group(10))
false
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is_finitelygeneratedFunction
is_finitelygenerated(G)

Return whether G is a finitely generated group.

Examples

julia> F = free_group(2)
<free group on the generators [ f1, f2 ]>

julia> is_finitelygenerated(F)
true

julia> H = derived_subgroup(F)[1]
Group(<free, no generators known>)

julia> is_finitelygenerated(H)
false
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Attributes of groups

orderMethod
order(::Type{T} = fmpz, x::Union{GAPGroupElem, GAPGroup}) where T <: IntegerUnion

Return the order of x, as an instance of T.

For a group element x in the group G, the order of x is the smallest positive integer n such that x^n is the identity of G. For a group x, the order of x is the number of elements in x.

An exception is thrown if the order of x is infinite, use isfinite in order to check for finiteness.

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exponentMethod
exponent(::Type{T} = fmpz, G::GAPGroup) where T <: IntegerUnion

Return the exponent of G, as an instance of T, i.e., the smallest positive integer $e$ such that $g^e$ is the identity of G for every $g$ in G.

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describeMethod
describe(G::GAPGroup)

Return a string that describes some aspects of the structure of G.

For finite groups, the function works well if G is an abelian group or a finite simple group or a group in one of the following series: symmetric, dihedral, quasidihedral, generalized quaternion, general linear, special linear.

For other finite groups, the function tries to decompose G as a direct product or a semidirect product, and if this is not possible as a non-splitting extension of a normal subgroup $N$ with the factor group G$/N$, where $N$ is the center or the derived subgroup or the Frattini subgroup of G.

For infinite groups, if the group is known to be finitely generated and abelian or free, a reasonable description is printed.

For general infinite groups, or groups for which finiteness is not (yet) known, not much if anything can be done. In particular we avoid potentially expensive checks such as computing the size of the group or whether it is abelian. While we do attempt a few limited fast checks for finiteness and commutativity, these will not detect all finite or commutative groups.

Thus calling describe again on the same group after additional information about it becomes known to Oscar may yield different outputs.

Note
  • for finitely presented groups, even deciding if the group is trivial is impossible in general; the same holds for most other properties, like whether the group is finite, abelian, etc.,
  • there is in general no "nice" decomposition of G,
  • there may be several decompositions of G,
  • nonisomorphic groups may yield the same describe result,
  • isomorphic groups may yield different describe results,
  • the computations can take a long time (for example in the case of large $p$-groups), and the results are still often not very helpful.

The following notation is used in the returned string.

DescriptionSyntax
trivial group1
finite cyclic groupC<size>
infinite cyclic groupZ
alternating groupA<degree>
symmetric groupS<degree>
dihedral groupD<size>
quaternion groupQ<size>
quasidihedral groupQD<size>
projective special linear groupPSL(<n>,<q>)
special linear groupSL(<n>,<q>)
general linear groupGL(<n>,<q>)
proj. special unitary groupPSU(<n>,<q>)
orthogonal group, type BO(2<n>+1,<q>)
orthogonal group, type DO+(2<n>,<q>)
orthogonal group, type 2DO-(2<n>,<q>)
proj. special symplectic groupPSp(2<n>,<q>)
Suzuki group (type 2B)Sz(<q>)
Ree group (type 2F or 2G)Ree(<q>)
Lie group of exceptional typeE(6,<q>), E(7,<q>), E(8,<q>), 2E(6,<q>), F(4,<q>), G(2,<q>)
Steinberg triality group3D(4,<q>)
sporadic simple groupM11, M12, M22, M23, M24, J1, J2, J3, J4, Co1, Co2, Co3, Fi22, Fi23, Fi24', Suz, HS, McL, He, HN, Th, B, M, ON, Ly, Ru
Tits group2F(4,2)'
the indicated group from the library of perfect groupsPerfectGroup(<size>,<id>)
direct productA x B
semidirect productN : H
non-split extensionZ(G) . G/Z(G) = G' . G/G', Phi(G) . G/Phi(G)

Examples

julia> g = symmetric_group(6);

julia> describe(g)
"S6"

julia> describe(sylow_subgroup(g,2)[1])
"C2 x D8"

julia> describe(sylow_subgroup(g, 3)[1])
"C3 x C3"

julia> describe(free_group(3))
"a free group of rank 3"
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nilpotency_classMethod
nilpotency_class(G::GAPGroup) -> Int

Return the nilpotency class of G, i.e., the smallest integer $n$ such that G has a central series of length $n$.

An exception is thrown if G is not nilpotent.

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prime_of_pgroupFunction
prime_of_pgroup(::Type{T} = fmpz, G::GAPGroup) where T <: IntegerUnion

Return the prime $p$ if G is a non-trivial $p$-group.

An exception is thrown if G is not a $p$-group or is a trivial group.

Examples

julia> prime_of_pgroup(quaternion_group(8))
2

julia> prime_of_pgroup(UInt16, quaternion_group(8))
0x0002

julia> prime_of_pgroup(symmetric_group(1))
ERROR: only supported for non-trivial p-groups

julia> prime_of_pgroup(symmetric_group(3))
ERROR: only supported for non-trivial p-groups
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