Representation theory¶
Ordinary characters¶
How can you compute character tables of a finite group in Sage? The Sage-GAP interface can be used to compute character tables.
You can construct the table of character values of a permutation
group \(G\) as a Sage matrix, using the method
character_table of the PermutationGroup class, or via the
interface to the GAP command CharacterTable.
sage: G = PermutationGroup([[(1,2),(3,4)], [(1,2,3,4)]])
sage: G.order()
8
sage: G.character_table()
[ 1 1 1 1 1]
[ 1 -1 -1 1 1]
[ 1 -1 1 -1 1]
[ 1 1 -1 -1 1]
[ 2 0 0 0 -2]
sage: CT = libgap(G).CharacterTable()
sage: CT.Display()
CT1
2 3 2 2 2 3
1a 2a 2b 4a 2c
2P 1a 1a 1a 2c 1a
3P 1a 2a 2b 4a 2c
X.1 1 1 1 1 1
X.2 1 -1 -1 1 1
X.3 1 -1 1 -1 1
X.4 1 1 -1 -1 1
X.5 2 . . . -2
>>> from sage.all import *
>>> G = PermutationGroup([[(Integer(1),Integer(2)),(Integer(3),Integer(4))], [(Integer(1),Integer(2),Integer(3),Integer(4))]])
>>> G.order()
8
>>> G.character_table()
[ 1 1 1 1 1]
[ 1 -1 -1 1 1]
[ 1 -1 1 -1 1]
[ 1 1 -1 -1 1]
[ 2 0 0 0 -2]
>>> CT = libgap(G).CharacterTable()
>>> CT.Display()
CT1
<BLANKLINE>
2 3 2 2 2 3
<BLANKLINE>
1a 2a 2b 4a 2c
2P 1a 1a 1a 2c 1a
3P 1a 2a 2b 4a 2c
<BLANKLINE>
X.1 1 1 1 1 1
X.2 1 -1 -1 1 1
X.3 1 -1 1 -1 1
X.4 1 1 -1 -1 1
X.5 2 . . . -2
Here is another example:
sage: G = PermutationGroup([[(1,2),(3,4)], [(1,2,3)]])
sage: G.character_table()
[ 1 1 1 1]
[ 1 -zeta3 - 1 zeta3 1]
[ 1 zeta3 -zeta3 - 1 1]
[ 3 0 0 -1]
sage: G = libgap.eval("Group((1,2)(3,4),(1,2,3))"); G
Group([ (1,2)(3,4), (1,2,3) ])
sage: T = G.CharacterTable()
sage: T.Display()
CT2
2 2 . . 2
3 1 1 1 .
1a 3a 3b 2a
2P 1a 3b 3a 1a
3P 1a 1a 1a 2a
X.1 1 1 1 1
X.2 1 A /A 1
X.3 1 /A A 1
X.4 3 . . -1
A = E(3)^2
= (-1-Sqrt(-3))/2 = -1-b3
>>> from sage.all import *
>>> G = PermutationGroup([[(Integer(1),Integer(2)),(Integer(3),Integer(4))], [(Integer(1),Integer(2),Integer(3))]])
>>> G.character_table()
[ 1 1 1 1]
[ 1 -zeta3 - 1 zeta3 1]
[ 1 zeta3 -zeta3 - 1 1]
[ 3 0 0 -1]
>>> G = libgap.eval("Group((1,2)(3,4),(1,2,3))"); G
Group([ (1,2)(3,4), (1,2,3) ])
>>> T = G.CharacterTable()
>>> T.Display()
CT2
<BLANKLINE>
2 2 . . 2
3 1 1 1 .
<BLANKLINE>
1a 3a 3b 2a
2P 1a 3b 3a 1a
3P 1a 1a 1a 2a
<BLANKLINE>
X.1 1 1 1 1
X.2 1 A /A 1
X.3 1 /A A 1
X.4 3 . . -1
<BLANKLINE>
A = E(3)^2
= (-1-Sqrt(-3))/2 = -1-b3
where \(E(3)\) denotes a cube root of unity, \(ER(-3)\)
denotes a square root of \(-3\), say \(i\sqrt{3}\), and
\(b3 = \frac{1}{2}(-1+i \sqrt{3})\). Note the added print
Python command. This makes the output look much nicer.
sage: irr = G.Irr(); irr
[ Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, E(3)^2, E(3), 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, E(3), E(3)^2, 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 3, 0, 0, -1 ] ) ]
sage: irr.Display()
[ [ 1, 1, 1, 1 ],
[ 1, E(3)^2, E(3), 1 ],
[ 1, E(3), E(3)^2, 1 ],
[ 3, 0, 0, -1 ] ]
sage: CG = G.ConjugacyClasses(); CG
[ ()^G, (2,3,4)^G, (2,4,3)^G, (1,2)(3,4)^G ]
sage: gamma = CG[2]; gamma
(2,4,3)^G
sage: g = gamma.Representative(); g
(2,4,3)
sage: chi = irr[1]; chi
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, E(3)^2, E(3), 1 ] )
sage: g^chi
E(3)
>>> from sage.all import *
>>> irr = G.Irr(); irr
[ Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, E(3)^2, E(3), 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, E(3), E(3)^2, 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 3, 0, 0, -1 ] ) ]
>>> irr.Display()
[ [ 1, 1, 1, 1 ],
[ 1, E(3)^2, E(3), 1 ],
[ 1, E(3), E(3)^2, 1 ],
[ 3, 0, 0, -1 ] ]
>>> CG = G.ConjugacyClasses(); CG
[ ()^G, (2,3,4)^G, (2,4,3)^G, (1,2)(3,4)^G ]
>>> gamma = CG[Integer(2)]; gamma
(2,4,3)^G
>>> g = gamma.Representative(); g
(2,4,3)
>>> chi = irr[Integer(1)]; chi
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, E(3)^2, E(3), 1 ] )
>>> g**chi
E(3)
This last quantity is the value of the character chi at the group
element g.
Alternatively, if you turn IPython “pretty printing” off, then the table prints nicely.
sage: %Pprint
Pretty printing has been turned OFF
sage: G = libgap.eval("Group((1,2)(3,4),(1,2,3))"); G
Group([ (1,2)(3,4), (1,2,3) ])
sage: T = G.CharacterTable(); T
CharacterTable( Alt( [ 1 .. 4 ] ) )
sage: T.Display()
CT3
2 2 2 . .
3 1 . 1 1
1a 2a 3a 3b
2P 1a 1a 3b 3a
3P 1a 2a 1a 1a
X.1 1 1 1 1
X.2 1 1 A /A
X.3 1 1 /A A
X.4 3 -1 . .
A = E(3)^2
= (-1-Sqrt(-3))/2 = -1-b3
sage: irr = G.Irr(); irr
[ Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3)^2, E(3) ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3), E(3)^2 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 3, -1, 0, 0 ] ) ]
sage: irr.Display()
[ [ 1, 1, 1, 1 ],
[ 1, 1, E(3)^2, E(3) ],
[ 1, 1, E(3), E(3)^2 ],
[ 3, -1, 0, 0 ] ]
sage: %Pprint
Pretty printing has been turned ON
>>> from sage.all import *
>>> %Pprint
Pretty printing has been turned OFF
>>> G = libgap.eval("Group((1,2)(3,4),(1,2,3))"); G
Group([ (1,2)(3,4), (1,2,3) ])
>>> T = G.CharacterTable(); T
CharacterTable( Alt( [ 1 .. 4 ] ) )
>>> T.Display()
CT3
<BLANKLINE>
2 2 2 . .
3 1 . 1 1
<BLANKLINE>
1a 2a 3a 3b
2P 1a 1a 3b 3a
3P 1a 2a 1a 1a
<BLANKLINE>
X.1 1 1 1 1
X.2 1 1 A /A
X.3 1 1 /A A
X.4 3 -1 . .
<BLANKLINE>
A = E(3)^2
= (-1-Sqrt(-3))/2 = -1-b3
>>> irr = G.Irr(); irr
[ Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3)^2, E(3) ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3), E(3)^2 ] ),
Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 3, -1, 0, 0 ] ) ]
>>> irr.Display()
[ [ 1, 1, 1, 1 ],
[ 1, 1, E(3)^2, E(3) ],
[ 1, 1, E(3), E(3)^2 ],
[ 3, -1, 0, 0 ] ]
>>> %Pprint
Pretty printing has been turned ON
Brauer characters¶
The Brauer character tables in GAP do not yet have a “native”
interface. To access them you can directly interface with GAP using
the libgap.eval command.
The example below using the GAP interface illustrates the syntax.
sage: G = libgap.eval("Group((1,2)(3,4),(1,2,3))"); G
Group([ (1,2)(3,4), (1,2,3) ])
sage: irr = G.IrreducibleRepresentations(GF(7)); irr # random arch. dependent output
[ [ (1,2)(3,4), (1,2,3) ] -> [ [ [ Z(7)^0 ] ], [ [ Z(7)^4 ] ] ],
[ (1,2)(3,4), (1,2,3) ] -> [ [ [ Z(7)^0 ] ], [ [ Z(7)^2 ] ] ],
[ (1,2)(3,4), (1,2,3) ] -> [ [ [ Z(7)^0 ] ], [ [ Z(7)^0 ] ] ],
[ (1,2)(3,4), (1,2,3) ] ->
[ [ [ Z(7)^2, Z(7)^5, Z(7) ], [ Z(7)^3, Z(7)^2, Z(7)^3 ],
[ Z(7), Z(7)^5, Z(7)^2 ] ],
[ [ 0*Z(7), Z(7)^0, 0*Z(7) ], [ 0*Z(7), 0*Z(7), Z(7)^0 ],
[ Z(7)^0, 0*Z(7), 0*Z(7) ] ] ] ]
sage: brvals = [[chi.Image(c.Representative()).BrauerCharacterValue()
....: for c in G.ConjugacyClasses()] for chi in irr]
sage: brvals # random architecture dependent output
[ [ 1, 1, E(3)^2, E(3) ],
[ 1, 1, E(3), E(3)^2 ],
[ 1, 1, 1, 1 ],
[ 3, -1, 0, 0 ] ]
sage: T = G.CharacterTable()
sage: T.Display()
CT3
2 2 . . 2
3 1 1 1 .
1a 3a 3b 2a
2P 1a 3b 3a 1a
3P 1a 1a 1a 2a
X.1 1 1 1 1
X.2 1 A /A 1
X.3 1 /A A 1
X.4 3 . . -1
A = E(3)^2
= (-1-Sqrt(-3))/2 = -1-b3
>>> from sage.all import *
>>> G = libgap.eval("Group((1,2)(3,4),(1,2,3))"); G
Group([ (1,2)(3,4), (1,2,3) ])
>>> irr = G.IrreducibleRepresentations(GF(Integer(7))); irr # random arch. dependent output
[ [ (1,2)(3,4), (1,2,3) ] -> [ [ [ Z(7)^0 ] ], [ [ Z(7)^4 ] ] ],
[ (1,2)(3,4), (1,2,3) ] -> [ [ [ Z(7)^0 ] ], [ [ Z(7)^2 ] ] ],
[ (1,2)(3,4), (1,2,3) ] -> [ [ [ Z(7)^0 ] ], [ [ Z(7)^0 ] ] ],
[ (1,2)(3,4), (1,2,3) ] ->
[ [ [ Z(7)^2, Z(7)^5, Z(7) ], [ Z(7)^3, Z(7)^2, Z(7)^3 ],
[ Z(7), Z(7)^5, Z(7)^2 ] ],
[ [ 0*Z(7), Z(7)^0, 0*Z(7) ], [ 0*Z(7), 0*Z(7), Z(7)^0 ],
[ Z(7)^0, 0*Z(7), 0*Z(7) ] ] ] ]
>>> brvals = [[chi.Image(c.Representative()).BrauerCharacterValue()
... for c in G.ConjugacyClasses()] for chi in irr]
>>> brvals # random architecture dependent output
[ [ 1, 1, E(3)^2, E(3) ],
[ 1, 1, E(3), E(3)^2 ],
[ 1, 1, 1, 1 ],
[ 3, -1, 0, 0 ] ]
>>> T = G.CharacterTable()
>>> T.Display()
CT3
<BLANKLINE>
2 2 . . 2
3 1 1 1 .
<BLANKLINE>
1a 3a 3b 2a
2P 1a 3b 3a 1a
3P 1a 1a 1a 2a
<BLANKLINE>
X.1 1 1 1 1
X.2 1 A /A 1
X.3 1 /A A 1
X.4 3 . . -1
<BLANKLINE>
A = E(3)^2
= (-1-Sqrt(-3))/2 = -1-b3