$k$-Schur Functions

class sage.combinat.sf.new_kschur.KBoundedSubspace(Sym, k, t='t')

Bases: UniqueRepresentation, Parent

This class implements the subspace of the ring of symmetric functions spanned by $\{s_{\lambda }[X/(1-t)]{\}}_{{\lambda }_1\le k}=\{s_{\lambda }^{(k)}[X;t]{\}}_{{\lambda }_1\le k}$ over the base ring $\mathbf{Q}[t]$. When $t=1$, this space is in fact a subring of the ring of symmetric functions generated by the complete homogeneous symmetric functions $h_i$ for $1\le i\le k$.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: KB = Sym.kBoundedSubspace(3,1); KB
3-bounded Symmetric Functions over Rational Field with t=1

sage: Sym = SymmetricFunctions(QQ['t'])
sage: KB = Sym.kBoundedSubspace(3); KB
3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field

The $k$-Schur function basis can be constructed as follows:

sage: ks = KB.kschur(); ks
3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 3-Schur basis

K_kschur()

Return the $k$-bounded basis called the K-$k$-Schur basis.

See [Morse11] and [LamSchillingShimozono10].

REFERENCES:

[Morse11]

J. Morse, Combinatorics of the K-theory of affine Grassmannians, Adv. in Math., Volume 229, Issue 5, pp. 2950–2984.

[LamSchillingShimozono10]

T. Lam, A. Schilling, M.Shimozono, K-theory Schubert calculus of the affine Grassmannian, Compositio Math. 146 (2010), 811-852.

EXAMPLES:

sage: kB = SymmetricFunctions(QQ).kBoundedSubspace(3,1)
sage: g = kB.K_kschur()
sage: g
3-bounded Symmetric Functions over Rational Field with t=1 in the K-3-Schur basis
sage: kB = SymmetricFunctions(QQ['t']).kBoundedSubspace(3)
sage: g = kB.K_kschur()
Traceback (most recent call last):
...
ValueError: This basis only exists for t=1

khomogeneous()

The homogeneous basis of this algebra.

See also

kHomogeneous()

EXAMPLES:

sage: kh3 = SymmetricFunctions(QQ).kBoundedSubspace(3,1).khomogeneous()
sage: TestSuite(kh3).run()

kschur()

The $k$-Schur basis of this algebra.

See also

kSchur()

EXAMPLES:

sage: ks3 = SymmetricFunctions(QQ).kBoundedSubspace(3,1).kschur()
sage: TestSuite(ks3).run()

ksplit()

The $k$-split basis of this algebra.

See also

kSplit()

EXAMPLES:

sage: ksp3 = SymmetricFunctions(QQ).kBoundedSubspace(3,1).ksplit()
sage: TestSuite(ksp3).run()

realizations()

A list of realizations of this algebra.

EXAMPLES:

sage: SymmetricFunctions(QQ).kBoundedSubspace(3,1).realizations()
[3-bounded Symmetric Functions over Rational Field with t=1 in the 3-Schur basis,
 3-bounded Symmetric Functions over Rational Field with t=1 in the 3-split basis,
 3-bounded Symmetric Functions over Rational Field with t=1 in the 3-bounded homogeneous basis,
 3-bounded Symmetric Functions over Rational Field with t=1 in the K-3-Schur basis]
sage: SymmetricFunctions(QQ['t']).kBoundedSubspace(3).realizations()
[3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 3-Schur basis,
 3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 3-split basis]

retract(sym)

Return the retract of sym from the ring of symmetric functions to self.

INPUT:

• sym – a symmetric function

OUTPUT:

• the analogue of the symmetric function in the $k$-bounded subspace (if possible)

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: s = Sym.schur()
sage: KB = Sym.kBoundedSubspace(3,1); KB
3-bounded Symmetric Functions over Rational Field with t=1
sage: KB.retract(s[2]+s[3])
ks3[2] + ks3[3]
sage: KB.retract(s[2,1,1])
Traceback (most recent call last):
...
ValueError: s[2, 1, 1] is not in the image

class sage.combinat.sf.new_kschur.KBoundedSubspaceBases(base, t='t')

Bases: Category_realization_of_parent

The category of bases for the $k$-bounded subspace of symmetric functions.

class ElementMethods

Bases: object

expand(*args, **kwargs)

Return the monomial expansion of self in $n$ variables.

INPUT:

• n – positive integer

OUTPUT: monomial expansion of self in $n$ variables

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: ks = Sym.kschur(3,1)
sage: ks[3,1].expand(2)
x0^4 + 2*x0^3*x1 + 2*x0^2*x1^2 + 2*x0*x1^3 + x1^4
sage: s = Sym.schur()
sage: ks[3,1].expand(2) == s(ks[3,1]).expand(2)
True

sage: Sym = SymmetricFunctions(QQ['t'])
sage: ks = Sym.kschur(3)
sage: f = ks[3,2]-ks[1]
sage: f.expand(2)
t^2*x0^5 + (t^2 + t)*x0^4*x1 + (t^2 + t + 1)*x0^3*x1^2 + (t^2 + t + 1)*x0^2*x1^3 + (t^2 + t)*x0*x1^4 + t^2*x1^5 - x0 - x1

hl_creation_operator(nu, t=None)

This is the vertex operator that generalizes Jing’s operator.

It is a linear operator that raises the degree by $|\nu |$. This creation operator is a t-analogue of multiplication by s(nu) .

See also

Proposition 5 in [SZ2001].

INPUT:

• nu – a partition or a list of integers

• t – (default: None, in which case t is used) an element of the base ring

EXAMPLES:

sage: Sym = SymmetricFunctions(FractionField(QQ['t']))
sage: ks = Sym.kschur(4)
sage: s = Sym.schur()
sage: s(ks([3,1,1]).hl_creation_operator([1]))
(t-1)*s[2, 2, 1, 1] + t^2*s[3, 1, 1, 1] + (t^3+t^2-t)*s[3, 2, 1] + (t^3-t^2)*s[3, 3] + (t^4+t^3)*s[4, 1, 1] + t^4*s[4, 2] + t^5*s[5, 1]
sage: ks([3,1,1]).hl_creation_operator([1])
(t-1)*ks4[2, 2, 1, 1] + t^2*ks4[3, 1, 1, 1] + t^3*ks4[3, 2, 1] + (t^3-t^2)*ks4[3, 3] + t^4*ks4[4, 1, 1]

sage: Sym = SymmetricFunctions(QQ)
sage: ks = Sym.kschur(4,t=1)
sage: ks([3,1,1]).hl_creation_operator([1])
ks4[3, 1, 1, 1] + ks4[3, 2, 1] + ks4[4, 1, 1]

is_schur_positive(*args, **kwargs)

Return whether self is Schur positive.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: ks = Sym.kschur(3,1)
sage: f = ks[3,2]+ks[1]
sage: f.is_schur_positive()
True
sage: f = ks[3,2]-ks[1]
sage: f.is_schur_positive()
False

sage: Sym = SymmetricFunctions(QQ['t'])
sage: ks = Sym.kschur(3)
sage: f = ks[3,2]+ks[1]
sage: f.is_schur_positive()
True
sage: f = ks[3,2]-ks[1]
sage: f.is_schur_positive()
False

omega()

Return the $\omega$ operator on self.

At $t=1$, $\omega$ maps the $k$-Schur function $s_{\lambda }^{(k)}$ to $s_{{\lambda }^{(k)}}^{(k)}$, where ${\lambda }^{(k)}$ is the $k$-conjugate of the partition $\lambda$.

See also

k_conjugate().

For generic $t$, $\omega$ sends $s_{\lambda }^{(k)}[X;t]$ to $t^d{s}_{{\lambda }^{(k)}}^{(k)}[X;1/t]$, where $d$ is the size of the core of $\lambda$ minus the size of $\lambda$. Most of the time, this result is not in the $k$-bounded subspace.

See also

omega_t_inverse().

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: ks = Sym.kschur(3,1)
sage: ks[2,2,1,1].omega()
ks3[2, 2, 2]
sage: kh = Sym.khomogeneous(3)
sage: kh[3].omega()
h3[1, 1, 1] - 2*h3[2, 1] + h3[3]

sage: Sym = SymmetricFunctions(FractionField(QQ['t']))
sage: ks = Sym.kschur(3)
sage: ks[3,1,1].omega()
Traceback (most recent call last):
...
ValueError: t*s[2, 1, 1, 1] + s[3, 1, 1] is not in the image

omega_t_inverse()

Return the map $t\to 1/t$ composed with $\omega$ on self.

Unlike the map omega(), the result of omega_t_inverse() lives in the $k$-bounded subspace and hence will return an element even for generic $t$. For $t=1$, omega() and omega_t_inverse() return the same result.

EXAMPLES:

sage: Sym = SymmetricFunctions(FractionField(QQ['t']))
sage: ks = Sym.kschur(3)
sage: ks[3,1,1].omega_t_inverse()
1/t*ks3[2, 1, 1, 1]
sage: ks[3,2].omega_t_inverse()
1/t^2*ks3[1, 1, 1, 1, 1]

scalar(x, zee=None)

Return standard scalar product between self and x.

INPUT:

• x – element of the ring of symmetric functions over the same base ring as self

• zee – an optional function on partitions giving the value for the scalar product between $p_{\mu }$ and $p_{\mu }$ (default is to use the standard zee() function)

See also

scalar()

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ['t'])
sage: ks3 = Sym.kschur(3)
sage: ks3[3,2,1].scalar( ks3[2,2,2] )
t^3 + t
sage: dks3 = Sym.kBoundedQuotient(3).dks()
sage: [ks3[3,2,1].scalar(dks3(la)) for la in Partitions(6, max_part=3)]
[0, 1, 0, 0, 0, 0, 0]
sage: dks3 = Sym.kBoundedQuotient(3,t=1).dks()
sage: [ks3[2,2,2].scalar(dks3(la)) for la in Partitions(6, max_part=3)]
[0, t - 1, 0, 1, 0, 0, 0]
sage: ks3 = Sym.kschur(3,t=1)
sage: [ks3[2,2,2].scalar(dks3(la)) for la in Partitions(6, max_part=3)]
[0, 0, 0, 1, 0, 0, 0]
sage: kH = Sym.khomogeneous(4)
sage: kH([2,2,1]).scalar(ks3[2,2,1])
3

class ParentMethods

Bases: object

an_element()

Return an element of self.

EXAMPLES:

sage: SymmetricFunctions(QQ['t']).kschur(3).an_element()
2*ks3[] + 2*ks3[1] + 3*ks3[2]

antipode(element)

Return the antipode on self by lifting to the space of symmetric functions, computing the antipode, and then converting to self.parent(). This is only the antipode for $t=1$ and for other values of $t$ the result may not be in the space where the $k$-Schur functions live.

INPUT:

• element – an element in a basis of the ring of symmetric functions

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: ks3 = Sym.kschur(3,1)
sage: ks3[3,2].antipode()
-ks3[1, 1, 1, 1, 1]
sage: ks3.antipode(ks3[3,2])
-ks3[1, 1, 1, 1, 1]

coproduct(element)

Return the coproduct operation on element.

The coproduct is first computed on the homogeneous basis if $t=1$ and on the Hall-Littlewood Qp basis otherwise. The result is computed then converted to the tensor squared of self.parent().

INPUT:

• element – an element in a basis of the ring of symmetric functions

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: ks3 = Sym.kschur(3,1)
sage: ks3[2,1].coproduct()
ks3[] # ks3[2, 1] + ks3[1] # ks3[1, 1] + ks3[1] # ks3[2] + ks3[1, 1] # ks3[1] + ks3[2] # ks3[1] + ks3[2, 1] # ks3[]
sage: h3 = Sym.khomogeneous(3)
sage: h3[2,1].coproduct()
h3[] # h3[2, 1] + h3[1] # h3[1, 1] + h3[1] # h3[2] + h3[1, 1] # h3[1] + h3[2] # h3[1] + h3[2, 1] # h3[]
sage: ks3t = SymmetricFunctions(FractionField(QQ['t'])).kschur(3)
sage: ks3t[2,1].coproduct()
ks3[] # ks3[2, 1] + ks3[1] # ks3[1, 1] + ks3[1] # ks3[2] + ks3[1, 1] # ks3[1] + ks3[2] # ks3[1] + ks3[2, 1] # ks3[]
sage: ks3t[3,1].coproduct()
ks3[] # ks3[3, 1] + ks3[1] # ks3[2, 1] + (t+1)*ks3[1] # ks3[3] + ks3[1, 1] # ks3[2] + ks3[2] # ks3[1, 1]
+ (t+1)*ks3[2] # ks3[2] + ks3[2, 1] # ks3[1] + (t+1)*ks3[3] # ks3[1] + ks3[3, 1] # ks3[]
sage: h3.coproduct(h3[2,1])
h3[] # h3[2, 1] + h3[1] # h3[1, 1] + h3[1] # h3[2] + h3[1, 1] # h3[1] + h3[2] # h3[1] + h3[2, 1] # h3[]

counit(element)

Return the counit of element.

The counit is the constant term of element.

INPUT:

• element – an element in a basis of the ring of symmetric functions

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: ks3 = Sym.kschur(3,1)
sage: f = 2*ks3[2,1] + 3*ks3[[]]
sage: f.counit()
3
sage: ks3.counit(f)
3

degree_on_basis(b)

Return the degree of the basis element indexed by $b$.

INPUT:

• b – a partition

EXAMPLES:

sage: ks3 = SymmetricFunctions(QQ).kschur(3,1)
sage: ks3.degree_on_basis(Partition([3,2]))
5

one_basis()

Return the basis element indexing 1.

EXAMPLES:

sage: ks3 = SymmetricFunctions(QQ).kschur(3,1)
sage: ks3.one()  # indirect doctest
ks3[]

transition_matrix(other, n)

Return the degree n transition matrix between self and other.

INPUT:

• other – a basis in the ring of symmetric functions

• n – a positive integer

The entry in the $i^{th}$ row and $j^{th}$ column is the coefficient obtained by writing the $i^{th}$ element of the basis of self in terms of the basis other, and extracting the $j^{th}$ coefficient.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ); s = Sym.schur()
sage: ks3 = Sym.kschur(3,1)
sage: ks3.transition_matrix(s,5)
[1 1 1 0 0 0 0]
[0 1 0 1 0 0 0]
[0 0 1 0 1 0 0]
[0 0 0 1 0 1 0]
[0 0 0 0 1 1 1]

sage: Sym = SymmetricFunctions(QQ['t'])
sage: s = Sym.schur()
sage: ks = Sym.kschur(3)
sage: ks.transition_matrix(s,5)
[t^2   t   1   0   0   0   0]
[  0   t   0   1   0   0   0]
[  0   0   t   0   1   0   0]
[  0   0   0   t   0   1   0]
[  0   0   0   0 t^2   t   1]

super_categories()

The super categories of self.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ['t'])
sage: from sage.combinat.sf.new_kschur import KBoundedSubspaceBases
sage: KB = Sym.kBoundedSubspace(3)
sage: KBB = KBoundedSubspaceBases(KB); KBB
Category of k bounded subspace bases of 3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field
sage: KBB.super_categories()
[Category of realizations of 3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field,
 Join of Category of graded coalgebras with basis over Univariate Polynomial Ring in t over Rational Field
     and Category of subobjects of filtered modules with basis over Univariate Polynomial Ring in t over Rational Field]

class sage.combinat.sf.new_kschur.K_kSchur(kBoundedRing)

Bases: CombinatorialFreeModule

This class implements the basis of the $k$-bounded subspace called the K-$k$-Schur basis.

See [Morse2011], [LamSchillingShimozono2010].

REFERENCES:

[Morse2011]

J. Morse, Combinatorics of the K-theory of affine Grassmannians, Adv. in Math., Volume 229, Issue 5, pp. 2950–2984.

[LamSchillingShimozono2010]

T. Lam, A. Schilling, M.Shimozono, K-theory Schubert calculus of the affine Grassmannian, Compositio Math. 146 (2010), 811-852.

K_k_Schur_non_commutative_variables(la)

Return the K-$k$-Schur function, as embedded inside the affine zero Hecke algebra.

INPUT:

• la – A $k$-bounded Partition

OUTPUT:

• An element of the affine zero Hecke algebra.

EXAMPLES:

sage: g = SymmetricFunctions(QQ).kBoundedSubspace(3,1).K_kschur()
sage: g.K_k_Schur_non_commutative_variables([2,1])
T[3,1,0] + T[1,2,0] + T[3,2,0] + T[0,1,0] + T[2,0,1] + T[0,3,0] + T[2,0,3] + T[0,3,1] + T[2,3,2] + T[2,3,1] + T[3,1,2] + T[1,2,1] - T[2,0] - T[3,1]
sage: g.K_k_Schur_non_commutative_variables([])
1
sage: g.K_k_Schur_non_commutative_variables([4,1])
Traceback (most recent call last):
...
ValueError: Partition should be 3-bounded

homogeneous_basis_noncommutative_variables_zero_Hecke(la)

Return the homogeneous basis element indexed by la, viewed as an element inside the affine zero Hecke algebra. For the code, see method _homogeneous_basis.

INPUT:

• la – A $k$-bounded partition

OUTPUT:

• An element of the affine zero Hecke algebra.

EXAMPLES:

sage: g = SymmetricFunctions(QQ).kBoundedSubspace(3,1).K_kschur()
sage: g.homogeneous_basis_noncommutative_variables_zero_Hecke([2,1])
T[2,1,0] + T[3,1,0] + T[1,2,0] + T[3,2,0] + T[0,1,0] + T[2,0,1] + T[1,0,3] + T[0,3,0] + T[2,0,3] + T[0,3,2] + T[0,3,1] + T[2,3,2] + T[3,2,1] + T[2,3,1] + T[3,1,2] + T[1,2,1] - T[1,0] - 2*T[2,0] - T[0,3] - T[3,2] - 2*T[3,1] - T[2,1]
sage: g.homogeneous_basis_noncommutative_variables_zero_Hecke([])
1

lift(x)

Return the lift of a $k$-bounded symmetric function.

INPUT:

• x – An expression in the K-$k$-Schur basis. Equivalently, x can be a

  $k$-bounded partition (then x corresponds to the basis element indexed by x)

OUTPUT:

• A symmetric function.

EXAMPLES:

sage: g = SymmetricFunctions(QQ).kBoundedSubspace(3,1).K_kschur()
sage: g.lift([2,1])
h[2] + h[2, 1] - h[3]
sage: g.lift([])
h[]
sage: g.lift([4,1])
Traceback (most recent call last):
...
TypeError: do not know how to make x (= [4, 1]) an element of self (=3-bounded Symmetric Functions over Rational Field with t=1 in the K-3-Schur basis)

product(x, y)

Return the product of the two K-$k$-Schur functions.

INPUT:

• x, y – elements of the $k$-bounded subspace, in the K-$k$-Schur basis.

OUTPUT:

• An element of the $k$-bounded subspace, in the K-$k$-Schur basis

EXAMPLES:

sage: g = SymmetricFunctions(QQ).kBoundedSubspace(3,1).K_kschur()
sage: g.product(g([2,1]), g[1])
-2*Kks3[2, 1] + Kks3[2, 1, 1] + Kks3[2, 2]
sage: g.product(g([2,1]), g([]))
Kks3[2, 1]

retract(x)

Return the retract of a symmetric function.

INPUT:

• x – A symmetric function.

OUTPUT:

• A $k$-bounded symmetric function in the K-$k$-Schur basis.

EXAMPLES:

sage: g = SymmetricFunctions(QQ).kBoundedSubspace(3,1).K_kschur()
sage: m = SymmetricFunctions(QQ).m()
sage: g.retract(m[2,1])
-2*Kks3[1] + 4*Kks3[1, 1] - 2*Kks3[1, 1, 1] - Kks3[2] + Kks3[2, 1]
sage: g.retract(m([]))
Kks3[]

class sage.combinat.sf.new_kschur.kHomogeneous(kBoundedRing)

Bases: CombinatorialFreeModule

Space of $k$-bounded homogeneous symmetric functions.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: kH = Sym.khomogeneous(3)
sage: kH[2]
h3[2]
sage: kH[2].lift()
h[2]

class sage.combinat.sf.new_kschur.kSchur(kBoundedRing)

Bases: CombinatorialFreeModule

Space of $k$-Schur functions.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ['t'])
sage: KB = Sym.kBoundedSubspace(3); KB
3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field

The $k$-Schur function basis can be constructed as follows:

sage: ks3 = KB.kschur(); ks3
3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 3-Schur basis

We can convert to any basis of the ring of symmetric functions and, whenever it makes sense, also the other way round:

sage: s = Sym.schur()
sage: s(ks3([3,2,1]))
s[3, 2, 1] + t*s[4, 1, 1] + t*s[4, 2] + t^2*s[5, 1]
sage: t = Sym.base_ring().gen()
sage: ks3(s([3, 2, 1]) + t*s([4, 1, 1]) + t*s([4, 2]) + t^2*s([5, 1]))
ks3[3, 2, 1]
sage: s(ks3[2, 1, 1])
s[2, 1, 1] + t*s[3, 1]
sage: ks3(s[2, 1, 1] + t*s[3, 1])
ks3[2, 1, 1]

$k$-Schur functions are indexed by partitions with first part $\le k$. Constructing a $k$-Schur function for a larger partition raises an error:

sage: ks3([4,3,2,1]) #
Traceback (most recent call last):
...
TypeError: do not know how to make x (= [4, 3, 2, 1]) an element of self (=3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 3-Schur basis)

Similarly, attempting to convert a function that is not in the linear span of the $k$-Schur functions raises an error:

sage: ks3(s([4]))
Traceback (most recent call last):
...
ValueError: s[4] is not in the image

Note that the product of $k$-Schur functions is not guaranteed to be in the space spanned by the $k$-Schurs. In general, we only have that a $k$-Schur times a $j$-Schur function is in the $(k+j)$-bounded subspace. The multiplication of two $k$-Schur functions thus generally returns the product of the lift of the functions to the ambient symmetric function space. If the result happens to lie in the $k$-bounded subspace, then the result is cast into the $k$-Schur basis:

sage: ks2 = Sym.kBoundedSubspace(2).kschur()
sage: ks2[1] * ks2[1]
ks2[1, 1] + ks2[2]
sage: ks2[1] * ks2[2]
s[2, 1] + s[3]

Because the target space of the product of a $k$-Schur and a $j$-Schur has several possibilities, the product of a $k$-Schur and $j$-Schur function is not implemented for distinct $k$ and $j$. Let us show how to get around this ‘manually’:

sage: ks3 = Sym.kBoundedSubspace(3).kschur()
sage: ks2([2,1]) * ks3([3,1])
Traceback (most recent call last):
...
TypeError: unsupported operand parent(s) for *: '2-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 2-Schur basis' and '3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 3-Schur basis'

The workaround:

sage: f = s(ks2([2,1])) * s(ks3([3,1])); f # Convert to Schur functions first and multiply there.
s[3, 2, 1, 1] + s[3, 2, 2] + (t+1)*s[3, 3, 1] + s[4, 1, 1, 1]
+ (2*t+2)*s[4, 2, 1] + (t^2+t+1)*s[4, 3] + (2*t+1)*s[5, 1, 1]
+ (t^2+2*t+1)*s[5, 2] + (t^2+2*t)*s[6, 1] + t^2*s[7]

or:

sage: f = ks2[2,1].lift() * ks3[3,1].lift()
sage: ks5 = Sym.kBoundedSubspace(5).kschur()
sage: ks5(f) # The product of a 'ks2' with a 'ks3' is a 'ks5'.
ks5[3, 2, 1, 1] + ks5[3, 2, 2] + (t+1)*ks5[3, 3, 1] + ks5[4, 1, 1, 1]
+ (t+2)*ks5[4, 2, 1] + (t^2+t+1)*ks5[4, 3] + (t+1)*ks5[5, 1, 1] + ks5[5, 2]

For other technical reasons, taking powers of $k$-Schur functions is not implemented, even when the answer is still in the $k$-bounded subspace:

sage: ks2([1])^2
Traceback (most recent call last):
...
TypeError: unsupported operand parent(s) for ^: '2-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field in the 2-Schur basis' and 'Integer Ring'

Todo

Get rid of said technical “reasons”.

However, at $t=1$, the product of $k$-Schur functions is in the span of the $k$-Schur functions always. Below are some examples at $t=1$

sage: ks3 = Sym.kBoundedSubspace(3, t=1).kschur(); ks3
3-bounded Symmetric Functions over Univariate Polynomial Ring in t over Rational Field with t=1 in the 3-Schur basis
sage: s = SymmetricFunctions(ks3.base_ring()).schur()
sage: ks3(s([3]))
ks3[3]
sage: s(ks3([3,2,1]))
s[3, 2, 1] + s[4, 1, 1] + s[4, 2] + s[5, 1]
sage: ks3([2,1])^2    # taking powers works for t=1
ks3[2, 2, 1, 1] + ks3[2, 2, 2] + ks3[3, 1, 1, 1]

product_on_basis(left, right)

Take the product of two $k$-Schur functions.

If $t\ne 1$, then take the product by lifting to the Schur functions and then retracting back into the $k$-bounded subspace (if possible).

If $t=1$, then the product calls _product_on_basis_via_rectangles().

INPUT:

• left, right – partitions

OUTPUT:

• an element of the $k$-Schur functions

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ['t'])
sage: ks3 = Sym.kschur(3,1)
sage: kH = Sym.khomogeneous(3)
sage: ks3(kH[2,1,1])
ks3[2, 1, 1] + ks3[2, 2] + ks3[3, 1]
sage: ks3([])*kH[2,1,1]
ks3[2, 1, 1] + ks3[2, 2] + ks3[3, 1]
sage: ks3([3,3,3,2,2,1,1,1])^2
ks3[3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1]
sage: ks3([3,3,3,2,2,1,1,1])*ks3([2,2,2,2,2,1,1,1,1])
ks3[3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1]
sage: ks3([2,2,1,1,1,1])*ks3([2,2,2,1,1,1,1])
ks3[2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1] + ks3[2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1]
sage: ks3[2,1]^2
ks3[2, 2, 1, 1] + ks3[2, 2, 2] + ks3[3, 1, 1, 1]
sage: ks3 = Sym.kschur(3)
sage: ks3[2,1]*ks3[2,1]
s[2, 2, 1, 1] + s[2, 2, 2] + s[3, 1, 1, 1] + 2*s[3, 2, 1] + s[3, 3] + s[4, 1, 1] + s[4, 2]

class sage.combinat.sf.new_kschur.kSplit(kBoundedRing)

Bases: CombinatorialFreeModule

The $k$-split basis of the space of $k$-bounded-symmetric functions

Fix k a positive integer and t an element of the base ring.

The $k$-split functions are a basis for the space of $k$-bounded symmetric functions that also have the bases

$$\{Q_{\lambda }^{\prime }[X;t]{\}}_{{\lambda }_1\le k}=\{s_{\lambda }^{(k)}[X;t]{\}}_{{\lambda }_1\le k}$$

where $Q_{\lambda }^{\prime }[X;t]$ are the Hall-Littlewood symmetric functions (using the notation of [MAC]) and $s_{\lambda }^{(k)}[X;t]$ are the $k$-Schur functions. If $t$ is not a root of unity, then

$$\{s_{\lambda }[X/(1-t)]{\}}_{{\lambda }_1\le k}$$

is also a basis of this space.

The $k$-split basis has the property that $Q_{\lambda }^{\prime }[X;t]$ expands positively in the $k$-split basis and the $k$-split basis conjecturally expands positively in the $k$-Schur functions. See [LLMSSZ] p. 81.

The $k$-split basis is defined recursively using the Hall-Littlewood creation operator defined in [SZ2001]. If a partition la is the concatenation (as lists) of a partition mu and nu where mu has maximal hook length equal to k then ksp(la) = ksp(nu).hl_creation_operator(mu). If the hook length of la is less than or equal to k, then ksp(la) is equal to the Schur function indexed by la.

EXAMPLES:

sage: Symt = SymmetricFunctions(QQ['t'].fraction_field())
sage: kBS3 = Symt.kBoundedSubspace(3)
sage: ks3 = kBS3.kschur()
sage: ksp3 = kBS3.ksplit()
sage: ks3(ksp3[2,1,1])
ks3[2, 1, 1] + t*ks3[2, 2]
sage: ksp3(ks3[2,1,1])
ksp3[2, 1, 1] - t*ksp3[2, 2]
sage: ksp3[2,1]*ksp3[1]
s[2, 1, 1] + s[2, 2] + s[3, 1]
sage: ksp3[2,1].hl_creation_operator([1])
t*ksp3[2, 1, 1] + (-t^2+t)*ksp3[2, 2]

sage: Qp = Symt.hall_littlewood().Qp()
sage: ksp3(Qp[3,2,1])
ksp3[3, 2, 1] + t*ksp3[3, 3]

sage: kBS4 = Symt.kBoundedSubspace(4)
sage: ksp4 = kBS4.ksplit()
sage: ksp4(ksp3([3,2,1]))
ksp4[3, 2, 1] - t*ksp4[3, 3] + t*ksp4[4, 1, 1]
sage: ks4 = kBS4.kschur()
sage: ks4(ksp4[3,2,2,1])
ks4[3, 2, 2, 1] + t*ks4[3, 3, 1, 1] + t*ks4[3, 3, 2]