Path: blob/develop/src/doc/en/constructions/rep_theory.rst
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*********************
Representation theory
*********************
.. index:
pair: ordinary representation; character
.. _section-character:
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 :math:`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
<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
<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 :math:`E(3)` denotes a cube root of unity, :math:`ER(-3)`
denotes a square root of :math:`-3`, say :math:`i\sqrt{3}`, and
:math:`b3 = \frac{1}{2}(-1+i \sqrt{3})`. Note the added ``print``
Python command. This makes the output look much nicer.
.. link
::
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)
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.
.. skip
::
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
<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
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
.. index::
pair: modular representation; character
pair: character; Brauer
.. _section-brauer:
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
<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