The L3-U3-quotient algorithm
The L3-U3-quotient algorithm computes for a finitely presented group on two
generators all quotients isomorphic to PSL(3,q), PGL(3, q), PSU(3, q),
or PGU(3, q), simultaneously for all prime powers q.
A Magma implementation is available here: l3.tgz.
This includes an
algorithm to compute minimal associated primes.
The algorithm requires a Magma version >= 2.19.
To use the algorithm, after starting magma, type
> Attach("minass.magma");
> AttachSpec("l3.spec");
Note that the algorithm does not return images
onto the group PSL(3,2) = PSL(2,7).
Contents
Basic usage
L3Quotients
The main method is L3Quotients,
which takes a finitely presented group and computes all L3-quotients.
> G := Group< a,b | a^2, b^3, (a*b)^11, (a,b)^11 >;
> L3Quotients(G);
[
U_2(43)
]
> H := Group< a,b | a^2, b^3, (a*b)^11, (a,b)^28 >;
> L3Quotients(H);
[
U_3(43),
U_3(14057)
]
This means that G has PSU(3,43) as quotient, but no other PSL(3,q), PGL(3, q),
PSU(3, q), or PGU(3, q) is a quotient of G.
Similarly, the only L3- or U3-quotients of H are PSU(3, 43) and PSU(3, 14057).
> G := Group< a,b | a^2, b^3, (a*b)^18, (a,b)^16 >;
> L3Quotients(G);
[
PGU(3, 71),
U_3(1889),
PGU(3, 17),
PGU(3, 17),
PGU(3, 17),
PGL(3, 19)
]
In this example, PGU(3,17) occurs three times. This means that there are three
epimorphisms of G onto PGU(3,17) which do
not differ by an automorphism
of PGU(3,17). In other words, the kernels of the epimorphisms are distinct.
Some groups have infinitely many L3-quotients.
This is indicated by one of the L3-quotients L_3(infty^k), L_3(p^(infty^d)),
or L_3(infty^(infty^d)).
See below for an interpretation of this output, and how to use Magma to get more
information about the quotients.
> G := Group< a, b | a^2, b^3, (a*b)^9 >;
> L3Quotients(G);
[
L_3(infty^18)
]
> H := Group< a, b | a^2, b^3, (a,b)^5, (a, b*a*b*a*b)^3 >;
> L3Quotients(H);
[
L_3(2^infty)
]
> K := Group< a, b | a^2, b^3 >;
> L3Quotients(K);
[
L_3(infty^(infty^2))
]
Even without further interpretation of the output, this tells us that these
groups are all infinite.
GetMatrices
For a finite L3-U3-quotient of G, that is, a quotient L_3(p^k), PGL(3, p^k),
U_3(p^k), or PGU(3, p^k), we can compute the matrix images of the generators,
using
GetMatrices.
This method takes an L3-U3-quotient and returns a matrix group H generated by
two elements, corresponding to the generators of G.
> G := Group< a, b | a^2, b^3, (a*b)^11, (a,b)^28 >;
> quot := L3Quotients(G); quot;
[
U_3(43),
U_3(14057)
]
> H := GetMatrices(quot[2]);
> H;
MatrixGroup(3, GF(14057^2))
Generators:
[ 13479*$.1 + 8387 13479*$.1 + 8388 12480*$.1 + 1163]
[ 13727*$.1 + 1669 13727*$.1 + 1668 9677*$.1 + 9154]
[ 11763*$.1 + 3700 11763*$.1 + 3700 908*$.1 + 4001]
[ 562*$.1 + 11659 11987*$.1 + 12203 4961*$.1 + 13767]
[11922*$.1 + 10773 5054*$.1 + 6130 3695*$.1 + 3079]
[ 3814*$.1 + 8470 4617*$.1 + 9807 8441*$.1 + 10325]
Note that G -> H, G.i -> H.i does in general not define a homomorphism,
but the induced map G -> H/Z(H) does.
Intermediate usage
Specifying orders
We often are only interested in quotients where certain orders are satisfied
(for instance, we might know that the generator must have a certain order).
Usually this yields a great speed-up in the computation, or even allows the
computation to finish in the first place.
The orders can be specified using the optional parameter
exactOrders.
This is a list of pairs, where the first entry is a word in the group, and the
second entry is the order.
Advanced usage
There are several parameters to influence the run of
L3Quotients
exclude
The optional boolean parameter
exclude is a list of
primes (default:
[]).
The algorithm does not compute L3-quotients in characteristic p if p is in
exclude.
Parameters influencing minimal associated primes
The optional parameters
useRandomTechniques,
factorizationBound,
trialDivisionBound,
and
groebnerBasisBound
are passed to the method
MinimalAssociatedPrimes
(see documentation there).
Handling infinite L3-quotients
There are three types of infinite quotients,
L_3(infty^k), L_3(p^(infty^d)), and L_3(infty^(infty^d))
Quotients of type L_3(infty^k)
If G has a quotient L_3(infty^k), then for almost all (all but finitely many)
primes p, G has finitely many quotients of type PSL(3,p^r), PGL(3, p^(r/3)),
PSU(3, p^(r/2)), or PGU(3, p^(r/6) with r <= k.
So L_3(infty^k) is a mnemonic, where infty in the base stands for infinitely
many primes, and k stands for the highest possible exponent.
There are two basic methods to further investigate such quotients.
The first is
SpecifyCharacteristic,
which takes an L3-quotient and an integer n, and computes the L3-quotients
in characteristic p|n.
> G := Group< a, b | a^2, b^3, (a*b)^9 >;
> quot := L3Quotients(G); quot;
[
L_3(infty^18)
]
> Q := quot[1];
> SpecifyCharacteristic(Q, 103);
[
PGL(3, 103^3)
]
> SpecifyCharacteristic(Q, 107);
[
U_3(107),
U_3(107),
U_3(107)
]
> SpecifyCharacteristic(Q, 109);
[
L_3(109),
L_3(109),
L_3(109)
]
> SpecifyCharacteristic(Q, 113);
[
PGU(3, 113^3)
]
The second is
AddGroupRelations,
which takes an L3-quotient and a list of group elements interpreted as
relations, and computes the L3-quotients which satisfy these relations.
> G< a, b > := Group< a, b | a^2, b^3, (a*b)^11 >;
> quot := L3Quotients(G); quot;
[
L_3(infty^20)
]
> Q := quot[1];
> AddGroupRelations(Q, [(a,b)^32]);
[
L_3(353^2),
U_3(241)
]
Quotients of type L_3(p^(infty^d))
If G has a quotient L_3(p^(infty^d)), then there are infinitely many positive
integers k such that G has a quotient of type PSL(3,p^k), PGL(3, p^k),
PSU(3, p^k), or PGU(3,p^k).
So L_3(p^(infty^d)) is a mnemonic, where infty in the exponent stands for
infinitely many possible exponents.
The parameter d describes the degree of infinity, and is ommited if d = 1.
Again, we can use
AddGroupRelations
to sudy this quotient further.
> H< a, b > := Group< a, b | a^2, b^3, (a,b)^5, (a, b*a*b*a*b)^3 >;
> quot := L3Quotients(H); quot;
[
L_3(2^infty)
]
> Q := quot[1];
> AddGroupRelations(Q, [(a*b)^(2*3*5*7)]);
[
PGL(3, 2^2),
U_3(2^2)
]
Another possibility is to add further ring relations to the ideal describing the
L3-quotient, using the method
AddRingRelations.
It takes an L3-quotient and a list of polynomials, and computes the L3-quotients
whose traces satisfy these polynomial relations.
> H< a, b > := Group< a, b | a^2, b^3, (a,b)^5, (a, b*a*b*a*b)^3 >;
> quot := L3Quotients(H); quot;
[
L_3(2^infty)
]
> Q := quot[1];
> Q`Ideal;
Ideal of Graded Polynomial ring of rank 10 over Integer Ring
Order: Grevlex with weights [8, 2, 2, 2, 2, 4, 4, 4, 4, 1]
Variables: xcom, x1, xm1, x2, xm2, x12, xm12, xm21, xm2m1, zeta
Variable weights: [8, 2, 2, 2, 2, 4, 4, 4, 4, 1]
Inhomogeneous, Dimension >0
Groebner basis:
[
xcom + zeta + 1,
xm12*xm2m1 + zeta,
x12 + xm12,
xm21 + xm2m1,
x1 + 1,
xm1 + 1,
x2,
xm2,
zeta^2 + zeta + 1,
2
]
> R< xcom, x1, xm1, x2, xm2, x12, xm12, xm21, xm2m1, zeta > := Generic(Q`Ideal);
> AddRingRelations(Q, [x12^5 + xm21^2 + 1]);
[
L_3(2^8),
L_3(2^6)
]
Quotients of type L_2(infty^(infty^d))
If G has a quotient L_2(infty^(infty^d)), then for almost all primes p and
infinitely many positive integers k, G has a quotient of type PSL(3,p^k),
PGL(3, p^k), PSU(3, p^k), or PGU(3, p^k).
So L_3(infty^(infty^d)) is a mnemonic, where infty in the base stands for
infinitely many primes, and infty in the exponent stands for infinitely many
possible exponents.
The parameter d describes the degree of infinity, and is ommited if d = 1.
These quotients can be further investigated using the methods
AddGroupRelations,
AddRingRelations, and
SpecifyCharacteristic.
Verbose printing
Use
SetVerbose("L3Quotients", d) to set the verbose
printing for L3Quotients, where
d is a value between
0 and 3.
References
[1] |
S. Jambor
An L3-U3-quotient algorithm for finitely presented groups
PhD Thesis (2012).
|