AUTHORS:
Bases: sage.structure.unique_representation.UniqueRepresentation, sage.structure.parent.Parent
Symmetric functions in non-commutative variables.
The ring of symmetric functions in non-commutative variables,
which is not to be confused with the non-commutative symmetric
functions, is the ring of all
bounded-degree noncommutative power series in countably many
indeterminates (i.e., elements in
of bounded
degree) which are invariant with respect to the action of the
symmetric group
on the indices of the indeterminates.
It can be regarded as a direct limit over all
of rings
of
-invariant polynomials in
non-commuting variables
(that is,
-invariant elements of
).
This ring is implemented as a Hopf algebra whose basis elements are indexed by set parititions.
Let be a set partition of the integers
. A monomial basis element indexed by
represents the sum of monomials
where
if and only if
and
are in the same part
for some
.
The -th graded component of the ring of symmetric functions in
non-commutative variables has its dimension equal to the number of
set partitions of
. (If we work, instead, with finitely many –
say,
– variables, then its dimension is equal to the number of
set partitions of
where the number of parts is at most
.)
Note
All set partitions are considered standard, a set partition of
for some
, unless otherwise stated.
REFERENCES:
[BZ05] | N. Bergeron, M. Zabrocki. The Hopf algebra of symmetric functions and quasisymmetric functions in non-commutative variables are free and cofree. (2005). Arxiv math/0509265v3. |
[BHRZ06] | N. Bergeron, C. Hohlweg, M. Rosas, M. Zabrocki. Grothendieck bialgebras, partition lattices, and symmetric functions in noncommutative variables. Electronic Journal of Combinatorics. 13 (2006). |
[RS06] | M. Rosas, B. Sagan. Symmetric functions in noncommuting variables. Trans. Amer. Math. Soc. 358 (2006). no. 1, 215-232. Arxiv math/0208168. |
[BRRZ08] | N. Bergeron, C. Reutenauer, M. Rosas, M. Zabrocki. Invariants and coinvariants of the symmetric group in noncommuting variables. Canad. J. Math. 60 (2008). 266-296. http://www.lacim.uqam.ca/~christo/Publi%C3%A9s/2008/NBergeronRosasZabrocki.pdf |
EXAMPLES:
We begin by first creating the ring of and the bases that are
analogues of the usual symmetric functions:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: m = NCSym.m()
sage: e = NCSym.e()
sage: h = NCSym.h()
sage: p = NCSym.p()
sage: m
Symmetric functions in non-commuting variables over the Rational Field in the monomial basis
The basis is indexed by set partitions, so we create a few elements and convert them between these bases:
sage: elt = m(SetPartition([[1,3],[2]])) - 2*m(SetPartition([[1],[2]])); elt
-2*m{{1}, {2}} + m{{1, 3}, {2}}
sage: e(elt)
1/2*e{{1}, {2, 3}} - 2*e{{1, 2}} + 1/2*e{{1, 2}, {3}} - 1/2*e{{1, 2, 3}} - 1/2*e{{1, 3}, {2}}
sage: h(elt)
-4*h{{1}, {2}} - 2*h{{1}, {2}, {3}} + 1/2*h{{1}, {2, 3}} + 2*h{{1, 2}}
+ 1/2*h{{1, 2}, {3}} - 1/2*h{{1, 2, 3}} + 3/2*h{{1, 3}, {2}}
sage: p(elt)
-2*p{{1}, {2}} + 2*p{{1, 2}} - p{{1, 2, 3}} + p{{1, 3}, {2}}
sage: m(p(elt))
-2*m{{1}, {2}} + m{{1, 3}, {2}}
sage: elt = p(SetPartition([[1,3],[2]])) - 4*p(SetPartition([[1],[2]])) + 2; elt
2*p{} - 4*p{{1}, {2}} + p{{1, 3}, {2}}
sage: e(elt)
2*e{} - 4*e{{1}, {2}} + e{{1}, {2}, {3}} - e{{1, 3}, {2}}
sage: m(elt)
2*m{} - 4*m{{1}, {2}} - 4*m{{1, 2}} + m{{1, 2, 3}} + m{{1, 3}, {2}}
sage: h(elt)
2*h{} - 4*h{{1}, {2}} - h{{1}, {2}, {3}} + h{{1, 3}, {2}}
sage: p(m(elt))
2*p{} - 4*p{{1}, {2}} + p{{1, 3}, {2}}
There is also a shorthand for creating elements. We note that we must use p[[]] to create the empty set partition due to python’s syntax.
sage: eltm = m[[1,3],[2]] - 3*m[[1],[2]]; eltm
-3*m{{1}, {2}} + m{{1, 3}, {2}}
sage: elte = e[[1,3],[2]]; elte
e{{1, 3}, {2}}
sage: elth = h[[1,3],[2,4]]; elth
h{{1, 3}, {2, 4}}
sage: eltp = p[[1,3],[2,4]] + 2*p[[1]] - 4*p[[]]; eltp
-4*p{} + 2*p{{1}} + p{{1, 3}, {2, 4}}
There is also a natural projection to the usual symmetric functions by letting the variables commute. This projection map preserves the product and coproduct structure. We check that Theorem 2.1 of [RS06] holds:
sage: Sym = SymmetricFunctions(QQ)
sage: Sm = Sym.m()
sage: Se = Sym.e()
sage: Sh = Sym.h()
sage: Sp = Sym.p()
sage: eltm.to_symmetric_function()
-6*m[1, 1] + m[2, 1]
sage: Sm(p(eltm).to_symmetric_function())
-6*m[1, 1] + m[2, 1]
sage: elte.to_symmetric_function()
2*e[2, 1]
sage: Se(h(elte).to_symmetric_function())
2*e[2, 1]
sage: elth.to_symmetric_function()
4*h[2, 2]
sage: Sh(m(elth).to_symmetric_function())
4*h[2, 2]
sage: eltp.to_symmetric_function()
-4*p[] + 2*p[1] + p[2, 2]
sage: Sp(e(eltp).to_symmetric_function())
-4*p[] + 2*p[1] + p[2, 2]
Return the realization of the powersum basis of self.
OUTPUT:
EXAMPLES:
sage: SymmetricFunctionsNonCommutingVariables(QQ).a_realization()
Symmetric functions in non-commuting variables over the Rational Field in the powersum basis
Return the dual Hopf algebra of the symmetric functions in non-commuting variables.
EXAMPLES:
sage: SymmetricFunctionsNonCommutingVariables(QQ).dual()
Dual symmetric functions in non-commuting variables over the Rational Field
Bases: sage.combinat.ncsym.bases.NCSymBasis_abstract
The Hopf algebra of symmetric functions in non-commuting variables in the elementary basis.
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: e = NCSym.e()
alias of elementary.Element
Bases: sage.combinat.ncsym.bases.NCSymBasis_abstract
The Hopf algebra of symmetric functions in non-commuting variables in the homogeneous basis.
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: h = NCSym.h()
alias of homogeneous.Element
Bases: sage.combinat.ncsym.bases.NCSymBasis_abstract
The Hopf algebra of symmetric functions in non-commuting variables in the monomial basis.
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: m = NCSym.m()
alias of monomial.Element
Return the coproduct of a monomial basis element.
INPUT:
OUTPUT:
EXAMPLES:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).monomial()
sage: m[[1, 3], [2]].coproduct()
m{} # m{{1, 3}, {2}} + m{{1}} # m{{1, 2}} + m{{1, 2}} # m{{1}} + m{{1, 3}, {2}} # m{}
sage: m.coproduct_on_basis(SetPartition([]))
m{} # m{}
sage: m.coproduct_on_basis(SetPartition([[1,2,3]]))
m{} # m{{1, 2, 3}} + m{{1, 2, 3}} # m{}
sage: m[[1,5],[2,4],[3,7],[6]].coproduct()
m{} # m{{1, 5}, {2, 4}, {3, 7}, {6}} + m{{1}} # m{{1, 5}, {2, 4}, {3, 6}}
+ 2*m{{1, 2}} # m{{1, 3}, {2, 5}, {4}} + m{{1, 2}} # m{{1, 4}, {2, 3}, {5}}
+ 2*m{{1, 2}, {3}} # m{{1, 3}, {2, 4}} + m{{1, 3}, {2}} # m{{1, 4}, {2, 3}}
+ 2*m{{1, 3}, {2, 4}} # m{{1, 2}, {3}} + 2*m{{1, 3}, {2, 5}, {4}} # m{{1, 2}}
+ m{{1, 4}, {2, 3}} # m{{1, 3}, {2}} + m{{1, 4}, {2, 3}, {5}} # m{{1, 2}}
+ m{{1, 5}, {2, 4}, {3, 6}} # m{{1}} + m{{1, 5}, {2, 4}, {3, 7}, {6}} # m{}
Return the dual basis to the monomial basis.
OUTPUT:
EXAMPLES:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).m()
sage: m.dual_basis()
Dual symmetric functions in non-commuting variables over the Rational Field in the w basis
Compute the pairing between an element of self and an element of the dual.
INPUT:
OUTPUT:
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: m = NCSym.m()
sage: w = m.dual_basis()
sage: matrix([[m(A).duality_pairing(w(B)) for A in SetPartitions(3)] for B in SetPartitions(3)])
[1 0 0 0 0]
[0 1 0 0 0]
[0 0 1 0 0]
[0 0 0 1 0]
[0 0 0 0 1]
sage: (m[[1,2],[3]] + 3*m[[1,3],[2]]).duality_pairing(2*w[[1,3],[2]] + w[[1,2,3]] + 2*w[[1,2],[3]])
8
Return the image of the symmetric function f in self.
This is performed by converting to the monomial basis and extending the method sum_of_partitions() linearly. This is a linear map from the symmetric functions to the symmetric functions in non-commuting variables that does not preserve the product or coproduct structure of the Hopf algebra.
See also
to_symmetric_function()
INPUT:
OUTPUT:
EXAMPLES:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).m()
sage: mon = SymmetricFunctions(QQ).m()
sage: elt = m.from_symmetric_function(mon[2,1,1]); elt
1/12*m{{1}, {2}, {3, 4}} + 1/12*m{{1}, {2, 3}, {4}} + 1/12*m{{1}, {2, 4}, {3}}
+ 1/12*m{{1, 2}, {3}, {4}} + 1/12*m{{1, 3}, {2}, {4}} + 1/12*m{{1, 4}, {2}, {3}}
sage: elt.to_symmetric_function()
m[2, 1, 1]
sage: e = SymmetricFunctionsNonCommutingVariables(QQ).e()
sage: elm = SymmetricFunctions(QQ).e()
sage: e(m.from_symmetric_function(elm[4]))
1/24*e{{1, 2, 3, 4}}
sage: h = SymmetricFunctionsNonCommutingVariables(QQ).h()
sage: hom = SymmetricFunctions(QQ).h()
sage: h(m.from_symmetric_function(hom[4]))
1/24*h{{1, 2, 3, 4}}
sage: p = SymmetricFunctionsNonCommutingVariables(QQ).p()
sage: pow = SymmetricFunctions(QQ).p()
sage: p(m.from_symmetric_function(pow[4]))
p{{1, 2, 3, 4}}
sage: p(m.from_symmetric_function(pow[2,1]))
1/3*p{{1}, {2, 3}} + 1/3*p{{1, 2}, {3}} + 1/3*p{{1, 3}, {2}}
sage: p([[1,2]])*p([[1]])
p{{1, 2}, {3}}
Check that is the identity on
:
sage: all(m.from_symmetric_function(pow(la)).to_symmetric_function() == pow(la)
....: for la in Partitions(4))
True
Return the internal coproduct of a monomial basis element.
The internal coproduct is defined by
where we sum over all pairs of set partitions and
whose infimum is
.
INPUT:
OUTPUT:
EXAMPLES:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).monomial()
sage: m.internal_coproduct_on_basis(SetPartition([[1,3],[2]]))
m{{1, 2, 3}} # m{{1, 3}, {2}} + m{{1, 3}, {2}} # m{{1, 2, 3}} + m{{1, 3}, {2}} # m{{1, 3}, {2}}
The product on monomial basis elements.
The product of the basis elements indexed by two set partitions
and
is the sum of the basis elements indexed by set partitions
such that
where
and
. Here
is the infimum of
and
and
is the
pipe() operation.
Equivalently we can describe all
as matchings between the
partitions of
and
where if
is matched
with
, we take
instead of
and
in
.
INPUT:
OUTPUT:
EXAMPLES:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).monomial()
sage: A = SetPartition([[1], [2,3]])
sage: B = SetPartition([[1], [3], [2,4]])
sage: m.product_on_basis(A, B)
m{{1}, {2, 3}, {4}, {5, 7}, {6}} + m{{1}, {2, 3, 4}, {5, 7}, {6}}
+ m{{1}, {2, 3, 5, 7}, {4}, {6}} + m{{1}, {2, 3, 6}, {4}, {5, 7}}
+ m{{1, 4}, {2, 3}, {5, 7}, {6}} + m{{1, 4}, {2, 3, 5, 7}, {6}}
+ m{{1, 4}, {2, 3, 6}, {5, 7}} + m{{1, 5, 7}, {2, 3}, {4}, {6}}
+ m{{1, 5, 7}, {2, 3, 4}, {6}} + m{{1, 5, 7}, {2, 3, 6}, {4}}
+ m{{1, 6}, {2, 3}, {4}, {5, 7}} + m{{1, 6}, {2, 3, 4}, {5, 7}}
+ m{{1, 6}, {2, 3, 5, 7}, {4}}
sage: B = SetPartition([[1], [2]])
sage: m.product_on_basis(A, B)
m{{1}, {2, 3}, {4}, {5}} + m{{1}, {2, 3, 4}, {5}}
+ m{{1}, {2, 3, 5}, {4}} + m{{1, 4}, {2, 3}, {5}} + m{{1, 4}, {2, 3, 5}}
+ m{{1, 5}, {2, 3}, {4}} + m{{1, 5}, {2, 3, 4}}
sage: m.product_on_basis(A, SetPartition([]))
m{{1}, {2, 3}}
TESTS:
We check that we get all of the correct set partitions:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).monomial()
sage: A = SetPartition([[1], [2,3]])
sage: B = SetPartition([[1], [2]])
sage: S = SetPartition([[1,2,3], [4,5]])
sage: AB = SetPartition([[1], [2,3], [4], [5]])
sage: L = sorted(filter(lambda x: S.inf(x) == AB, SetPartitions(5)), key=str)
sage: map(list, L) == map(list, sorted(m.product_on_basis(A, B).support(), key=str))
True
Return the sum over all set partitions whose shape is la
with a fixed coefficient defined below.
Fix a partition , we define
and
.
Recall that
and
is the
number of parts of length
of
. Thus we defined the
coefficient as
Hence we can define a lift from
to
by
where the sum is over all set partitions whose shape
is .
INPUT:
OUTPUT:
EXAMPLES:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).m()
sage: m.sum_of_partitions(Partition([2,1,1]))
1/12*m{{1}, {2}, {3, 4}} + 1/12*m{{1}, {2, 3}, {4}} + 1/12*m{{1}, {2, 4}, {3}}
+ 1/12*m{{1, 2}, {3}, {4}} + 1/12*m{{1, 3}, {2}, {4}} + 1/12*m{{1, 4}, {2}, {3}}
TESTS:
Check that is the identity on
:
sage: m = SymmetricFunctionsNonCommutingVariables(QQ).m()
sage: mon = SymmetricFunctions(QQ).monomial()
sage: all(m.from_symmetric_function(mon[la]).to_symmetric_function() == mon[la]
....: for i in range(6) for la in Partitions(i))
True
Bases: sage.combinat.ncsym.bases.NCSymBasis_abstract
The Hopf algebra of symmetric functions in non-commuting variables in the powersum basis.
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: p = NCSym.p()
alias of powersum.Element
Return the result of the antipode applied to a powersum basis element.
Let be a set partition. The antipode given in [LM2011] is
where we sum over all ordered set partitions (i.e. set
compositions) of and
is the action of on
defined in
SetPartition.ordered_set_partition_action().
INPUT:
OUTPUT:
EXAMPLES:
sage: p = SymmetricFunctionsNonCommutingVariables(QQ).powersum()
sage: p.antipode_on_basis(SetPartition([[1], [2,3]]))
p{{1, 2}, {3}}
sage: p.antipode_on_basis(SetPartition([]))
p{}
sage: F = p[[1,3],[5],[2,4]].coproduct()
sage: F.apply_multilinear_morphism(lambda x,y: x.antipode()*y)
0
Return the coproduct of a monomial basis element.
INPUT:
OUTPUT:
EXAMPLES:
sage: p = SymmetricFunctionsNonCommutingVariables(QQ).powersum()
sage: p[[1, 3], [2]].coproduct()
p{} # p{{1, 3}, {2}} + p{{1}} # p{{1, 2}} + p{{1, 2}} # p{{1}} + p{{1, 3}, {2}} # p{}
sage: p.coproduct_on_basis(SetPartition([[1]]))
p{} # p{{1}} + p{{1}} # p{}
sage: p.coproduct_on_basis(SetPartition([]))
p{} # p{}
Return the internal coproduct of a powersum basis element.
The internal coproduct is defined by
INPUT:
OUTPUT:
EXAMPLES:
sage: p = SymmetricFunctionsNonCommutingVariables(QQ).powersum()
sage: p.internal_coproduct_on_basis(SetPartition([[1,3],[2]]))
p{{1, 3}, {2}} # p{{1, 3}, {2}}
Return the primitive associated to A in self.
Fix some . Let
be an atomic set partition of
,
then the primitive
given in [LM2011] is
where we sum over all ordered set partitions of such
that
and
is the action of
on
defined in
SetPartition.ordered_set_partition_action().
If
is not atomic, then
.
See also
INPUT:
OUTPUT:
EXAMPLES:
sage: p = SymmetricFunctionsNonCommutingVariables(QQ).powersum()
sage: elt = p.primitive(SetPartition([[1,3], [2]])); elt
-p{{1, 2}, {3}} + p{{1, 3}, {2}}
sage: elt.coproduct()
-p{} # p{{1, 2}, {3}} + p{} # p{{1, 3}, {2}} - p{{1, 2}, {3}} # p{} + p{{1, 3}, {2}} # p{}
sage: p.primitive(SetPartition([[1], [2,3]]))
0
sage: p.primitive(SetPartition([]))
p{}
Bases: sage.combinat.ncsym.bases.NCSymBasis_abstract
The Hopf algebra of symmetric functions in non-commuting variables
in the basis.
This basis was defined in [BZ05] as
where we sum over all strict coarsenings of the set partition .
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: q = NCSym.q()
Bases: sage.combinat.ncsym.bases.NCSymBasis_abstract
The Hopf algebra of symmetric functions in non-commuting variables
in the basis.
This basis is defined in [BHRZ06] by the formula:
and has the following properties:
EXAMPLES:
sage: NCSym = SymmetricFunctionsNonCommutingVariables(QQ)
sage: x = NCSym.x()
Iterate through all matchings of the sets and
.
EXAMPLES:
sage: from sage.combinat.ncsym.ncsym import matchings
sage: list(matchings([1, 2, 3], [-1, -2]))
[[[1], [2], [3], [-1], [-2]],
[[1], [2], [3, -1], [-2]],
[[1], [2], [3, -2], [-1]],
[[1], [2, -1], [3], [-2]],
[[1], [2, -1], [3, -2]],
[[1], [2, -2], [3], [-1]],
[[1], [2, -2], [3, -1]],
[[1, -1], [2], [3], [-2]],
[[1, -1], [2], [3, -2]],
[[1, -1], [2, -2], [3]],
[[1, -2], [2], [3], [-1]],
[[1, -2], [2], [3, -1]],
[[1, -2], [2, -1], [3]]]