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Abstract
We consider the core blocks corresponding to the Hecke algebras of type B over a field of arbitrary characteristic. To each core block, we associate two non-negative integers which determine the indexing of the Specht modules and simple modules in the block, the weight of the block, the multicharge of the algebra (up to a shift) and the block decomposition matrix.
KEYWORDS:
1 Introduction
Let denote an Ariki-Koike algebra over a field
of characteristic p with quantum characteristic
. Each of these algebras decomposes into a direct sum of indecomposable two-sided ideals, its blocks, and so in order to try to understand the algebras one may study certain types of block in the hope that they are more manageable.
There is an important class of -modules which are indexed by the r-multipartitions of n and which are known as Specht modules. The composition factors of each Specht module all lie in the same block and so we may think of partitioning the Specht modules into blocks. One may consider the block decomposition matrix, which records the composition factors of the Specht modules belonging to a given block. In general, computing block decomposition matrices is hard. There exist recursive algorithms which compute the transition coefficients for a highest weight module of the Fock space representation of
; by Ariki’s theorem [1], these coefficients coincide with the decomposition numbers for the Ariki-Koike algebras when p = 0. When p > 0, the transition coefficients give a lower bound for the decomposition numbers, and may be considered as a first approximation to them. It is reasonable to ask when this approximation is precise, or when the block decomposition matrix is independent of the characteristic of the field.
In this paper, we look at the core blocks for the Hecke algebras of type B, that is, the Ariki-Koike algebras . These core blocks, which were initially introduced and studied by Fayers [Citation11], are blocks in which none of the bipartitions indexing the Specht modules have any removable e-rim hooks. If
or e > n, all blocks are core blocks. To each core block B, we associate two non-negative integers nB and pB. We show that these two integers determine the indexing of the Specht modules and simple modules in the block, the weight of the block, the multicharge of
(up to a shift) and the block decomposition matrix. We present a result of George Witty [Citation27] that shows that in some situations they also determine the homomorphism space between pairs of Specht modules in the block. All these results are independent of the characteristic of the field.
When or e > n, decomposition numbers for the Hecke algebras of type B, or the corresponding cyclotomic q-Schur algbras, are not new. When
and the parameters of the multicharge are weakly increasing, Leclerc and Miyachi [Citation19, Theorem 3] give a closed formula for the canonical basis elements of the irreducible highest weight representation
; by Ariki’s theorem [1], this gives the graded decomposition numbers for the Hecke algebras when
and p = 0. When
and p = 0, Brundan and Stroppel [Citation7, Section 9] apply the theory they have developed earlier in their paper to construct a basis for the radical of their cell module
, thus determining the graded dimension of the simple modules. When
or e > n, Hu and Mathas [Citation14, Appendix B] give a formula for the graded decomposition numbers of the quiver Schur algebras of level two in terms of tableaux combinatorics; this is independent of the characteristic of the field. In current work [Citation2, Citation3], the authors determine full submodule structures for the Specht modules of the core blocks of the Hecke algebras and the Weyl modules for their Schur algebras.
Each paper above uses a different notation. In this paper, we index the (bipartitions corresponding to) Specht modules in a core block by sign sequences, sequences containing elements from with a fixed number of entries of each type, where the 0 entries are essentially redundant. The integers nB and pB above count the numbers of –s and + s respectively. The structure of the core blocks means that our combinatorics feels extremely natural. When
, it is straightforward to pass from our notation to that of [Citation19] or [Citation2].
The structure of this paper is as follows. In Section 2 we introduce the background material: Section 2.1 defines the relevant combinatorics and Section 2.2 introduces the Ariki-Koike algebras. For more information on the Ariki-Koike algebras and their combinatorics, we refer the reader to the survey paper of Mathas [Citation23] and for more information on their connections with the cyclotomic KLR algebras of type A, we refer them to the survey paper of Kleshchev [Citation17].
In Section 3, we move on to considering the core blocks, as introduced by Fayers [Citation11], when r = 2. In Section 3.1 we define the sign sequences and in Section 3.2 we show how they index the block decomposition matrices for core blocks. In Section 3.3 we briefly introduce the Fock space representation of and describe its connection with the Ariki-Koike algebras; more details can be found in the papers of Lascoux, Leclerc and Thibon [Citation18] and Ariki [Citation1]. In Section 3.4, we find the decomposition numbers for the core blocks. We start with the special case that the base tuple is flat, which includes the case that
, and then use what is essentially a focused version of Scopes equivalence [Citation8, Citation25] to generalize it to arbitrary core blocks. Finally, in Section 3.5 we summarize our results, present Witty’s theorem and give some examples.
2 Background
Throughout this paper, we will use a parameter . If e is finite, we define
which we may identify with
; otherwise we define
. In both cases we take < to be the usual total order on I.
2.1 Multipartitions and abacus configurations
Suppose that . A partition of n is a sequence
of non-negative integers such that
and
. We write
. We let Λn denote the set of partitions of n and
denote the set of all partitions. Now suppose that
. An r-multipartition, or multipartition, of n is an r-tuple of partitions
such that
. We write
. We let
denote the set of r-multipartitions of n and
denote the set of all r-multipartitions.
Suppose that . The Young diagram of
is the set
We say that a node is removable if
is the Young diagram of a multipartition and we say that
is addable if
is the Young diagram of a multipartition. The rim of
is the set of nodes
. For
, a removable h-rim hook is a set of h connected nodes in the rim such that removing those nodes gives the Young diagram of a multipartition.
Now fix . Suppose that
. To each node
we associate its residue
. If e is finite (resp.
) we set
(resp.
). For
we define the residue set of
to be the multiset
. We define an equivalence relation
on
by saying that
if and only if
and we refer to the
-equivalence classes of
as blocks. Clearly if e is finite and
with
for
then
if and only if
.
Given , we define a subset
. For
, we call an addable (resp. removable) node of residue i an addable (resp. removable) i-node. Given
and
, we define a total order
on the set of addable and removable nodes of
by saying that
if
or if k1 = k2 and
. We may then define the i-signature of
by looking at all the addable and removable i-nodes of
ordered according to
and writing
for an addable i-node and
for a removable i-node. We then construct the reduced i-signature of
by repeatedly removing all adjacent
-pairs until there are no such pairs left. If there are any
terms in the reduced i-signature of
, the first
term corresponds to a removable i-node of
which is called a good i-node.
It is usual to write + instead of and – instead of
, however we want use those symbols for other purposes in this paper. The set
is defined recursively. Suppose that
.
If
then
.
Otherwise, if
does not contain a good i-node for any
then
.
Otherwise, suppose that
is a good i-node of
for some
. Let
be the multipartition whose Young diagram is obtained from
by removing the node
. Then
if and only if
.
The multipartitions in are known as Kleshchev multipartitions; they have the property that if
then
is e-restricted for all
. We set
.
Given an r-multipartition, it is convenient to represent it as a r-tuple of abacus configurations. If e is finite, an e-abacus is an abacus with e vertical runners which are infinite in both directions and which are indexed from left to right by the elements of I. The possible bead positions are indexed by the elements of such that bead position b on the abacus is in row l of runner i where
and
. If
, the abacus has runners and bead positions indexed by the elements of
, so that runner
contains either one bead or no beads.
For and
, define the β-set
We define the abacus configuration of λ with respect to a to be the abacus where we put a bead at position b for each . For
and
we define the abacus configuration of
with respect to
to be an r-tuple of abacus configurations, the kth one of which is the abacus configuration of
with respect to ak.
Operations on the Young diagram of can be translated into operations on the abacus configuration of
.
Lemma 2.1.
Let and
. All residues below are with repect to
as are all abacus configurations.
Removing an i-node from component k of
corresponds to moving a bead on runner i of the abacus of
back by one position.
Adding an i-node to component k of
corresponds to moving a bead on runner i – 1 of the abacus of
forward by one position.
If
then adding (resp. removing) an e-rim hook to (resp. from) component k of
corresponds to pushing a bead on the abacus of
down one position (resp. up one position).
Example.
Take e = 3. Let and take
. Then the abacus configuration of
with respect to
is given by
and
has three removable e-rim hooks. For example,
is formed by
by removing an e-rim hook from
and has abacus configuration (with respect to
)
.
When drawing abacuses we truncate the runners and assume that the positions above the drawn portion all contain beads while the positions below are empty.
We say that is a multicore if
or if
and no component of
has any removable e-rim hooks; equivalently,
is a multicore if no bead in the abacus of any component has an empty space above it. (This is independent of the choice of
used to draw the abacus configurations.) We say that a
-equivalence class B is a core block if every
is a multicore. If
is a multicore and
then for
and
we define
as follows. If e is finite, set
in other words,
is the lowest row of the abacus configuration for
with respect to ak which contains a bead on runner i. If
, set
to be 1 if
and 0 otherwise.
Lemma 2.2.
[Citation11, Theorem 3.1] Let and suppose that B is a
-equivalence class.
Suppose e is finite. If B is a core block then there exists
, with
for all
, and
such that for each
and
is equal to either bi or
.
Suppose
. Then every block is a core block and if we set
and take
then for each
and
is equal to either bi or
.
Conversely, suppose that e is finite and that . If there exists
, with
for all
, and
such that for each
and
is equal to either bi or
, then the
-equivalence class of
is a core block.
2.2 The Ariki-Koike algebra
Let and
and let
be a field of characteristic
. Choose
and
. The Ariki-Koike algebra
is the unital associative
-algebra with generators
and relations
Define to be minimal such that
, or set
if no such value exists. Two parameters Qk and Ql are q-connected if
for some
. Each Ariki-Koike algebra
is Morita equivalent to a direct sum of tensor products of smaller algebras whose parameters are all q-connected [Citation9] and so we will assume that all our parameters are q-connected, in fact, that they are all powers of q where
. If
satisfies
for all
then we call
a multicharge for
. If e is finite then
so there are infinitely many possible multicharges for
.
The algebra is a cellular algebra [Citation10, Citation13] with the cell modules indexed by the r-multipartitions of n. The cell module
indexed by the multipartition
is called a Specht module. Due to the properties of cellular algebras, all the composition factors of
lie in the same block, and so we can think of the Specht modules as being partitioned into blocks.
Proposition 2.3.
[Citation21, Theorem 2.11] Suppose that is a multicharge for
and that
. Then
and
lie in the same block of
if and only if
.
This result explains why we called the -equivalence classes blocks: two multipartitions of n are in the same class if and only if the corresponding Specht modules lie in the same block.
From the properties of cellular algebras, we know that there is a bilinear form on each cell module. If we define to be the radical of the Specht module
with respect to this bilinear form then if
is a multicharge for
then
if and only if
and so
is a complete set of non-isomorphic irreducible
-modules. Given
and
we define
to be the multiplicity of the simple module
as a composition factor of the Specht module
.
Brundan and Kleshchev [Citation4] have shown that the Ariki-Koike algebras are isomorphic to certain graded algebras defined by Khovanov and Lauda [Citation15, Citation16] and by Rouquier [Citation24]: the cyclotomic KLR algebras of type A. Through this isomorphism, we may think of as being graded. There is a grading on the Specht modules [Citation6], thus we can define
to be the graded multiplicity of the simple module
as a composition factor of the Specht module
; we recover the original decomposition number by setting v = 1. For more details, we refer the reader to the survey paper [Citation17].
We define the (graded) decomposition matrix of to be the matrix whose rows are indexed by the elements of
and whose columns are indexed by the elements of
with entries equal to
, for
and
. If B is a
-equivalence class, where
is such that
, we define the block decomposition matrix of B to be the submatrix of the decomposition matrix of
whose rows and columns are indexed only by elements of B.
3 Core blocks when r = 2
For the rest of this paper, we assume r = 2 so that our Specht modules are indexed by bipartitions and we shall refer to multicores as bicores. We fix and
. We shall take a block to mean an
-equivalence class of
and a core block to be a block B in which each
is a bicore.
Let be a field of characteristic
. Fix
where q is a primitive eth root of unity if e is finite and
for any
otherwise. Let
. If B is a block then there exists
such that
for all
and so we associate to B the block of the Hecke algebra
containing the Specht modules
for
. We denote this corresponding block of
by
so that we have a correspondence
between
-equivalence classes and 2-sided indecomposable ideals of the Hecke algebras
; we refer to both as blocks.
3.1 Sign sequences
Set
We say that are essentially the same if the two sequences obtained by removing all the 0s from each of them are the same.
For , we define sets
and
as follows.
Take
and
.
If there do not exist
with i < j and
and
, end the process. Return
and
.
Otherwise, choose
with i < j and
and
with the property that
for all i < m < j. Add (i, j) to
and set
. Go back to step (2).
Example.
Suppose that e = 19 and . We find it helpful to draw a diagram for δ as below.
Then
Set
Suppose . For each
, define
by setting
If for some
, we write
and set
. We write
if
for some
where S has the property that if
with
then
.
Example.
Let e = 8 and . We write δ as
so that
. Then we have
Informally, we can see that each element of the set is obtained by swapping some pairs
that lie at either end of an arc in the diagram of δ.
3.2 Indexing the block decomposition matrices corresponding to core blocks
Let B be a core block. Suppose e is finite. By Lemma 2.2, we can find and
with the property that
for k = 1, 2 and for any
we have
or
, for
and k = 1, 2. If
satisfy these conditions we call
a reduced pair for B. We define a total order
(which depends on the choice of
) on I by saying that
if bi < bj or if bi = bj and i < j. If
with
we take
be the permutation that sends j to ij for
. If
, we define
where
to be the unique reduced pair for B. We then take
to be the usual total order < on
and π to be the identity permutation on
.
For and
a reduced pair for B, define
by setting
for all
. Our choice of
ensures that
, where we abuse notation by identifying 1 with + and –1 with –.
If e is finite, the reduced pair is not uniquely determined by the conditions above. However we shall see that unless
,
and
are essentially the same for any reduced pairs
and
.
Example.
Take e = 7 and . Let B be the core block containing the bipartition
If is a reduced pair for B then there exists
such that
is of the form
.
Taking we have two choices for
, each of which gives a different ordering
:
However, the only choice comes when we look at the third runner on the abacus configurations, that is, when , and we note that the two choices for
are essentially the same.
Lemma 3.1.
[Citation11, Propn. 3.7] Suppose B is a core block and is a reduced pair for B. Let
and suppose
with
and
. Define
to be the bicore
with
Then and
Moreover, if then we may form
from
by repeatedly applying operations of the form sij for some i, j as above.
Corollary 3.2.
Suppose that B is a core block and is a reduced pair for B. Let
. Then
can be formed from
by permuting the entries equal to ±. Conversely, any sequence
formed by permuting the entries equal to ± in
is equal to
for some
.
Consequently, if and only if
or
.
Lemma 3.3.
Suppose e is finite and that B is a core block with . Suppose that
and
are reduced pairs for B with
and
. Let
.
There exists
with
for k = 1, 2.
The sequences
and
are essentially the same.
Proof.
Since
and
are both reduced pairs for B, there exist
such that
for k = 1, 2. As
, there exist
such that
so since
for all
, we must have d1 = d2.
Using part (1) we may assume that
for k = 1, 2. If we set
then
is a reduced pair for B and
. Let
so that
if and only if
. If
then we must have
. So restricting the order
to the set
we see that changing
to
permutes the elements of
while keeping the non-zero entries in the same order. Hence
and
are essentially the same.
□
If , Lemma 3.3 does not hold. If e is finite and
, assume that we choose the reduced pair
such that
and
. This choice is arbitrary; it is simply made so that for every element of a core block we have an expression
which is essentially independent of the choice of the reduced pair
. Recall that if
, there is a unique reduced pair for B. Unless we need to emphasize the reduced pair, we will henceforth write
instead of
.
Suppose that B is a core block. Take and hence define
Given Corollary 3.2 and the discussion above, these parameters are well-defined and independent of the choice of .
Lemma 3.4.
There exists such that if e is finite (resp.
) then
and
(resp.
and
).
Proof.
Suppose that is a reduced pair for B and let
. Suppose e is finite. Then by [Citation20, Lemma 2.2],
for k = 1, 2. Hence
Suppose . Choose
. Then
for k = 1, 2. Hence
□
In [Citation12, Section 2.1], Fayers introduced the weight of a multipartition. Two multipartitions in the same block have the same weight and so for a block B we define to be the weight of any multipartition belonging to B. We define the weight of the corresponding block
as
. We do not give Fayers’ definition here, but we note that, roughly speaking, the blocks
of small weight tend to be easier to understand. We have
if and only if
which holds if and only if
is simple.
Lemma 3.5.
[Citation12, Propn. 3.8] Suppose that B is a core block. Then .
Corollary 3.6.
Let B be a core block. If e is finite then
For a core block B, we have seen that the Specht modules in can be indexed by a subset of Δ. The next problem is to decide which
index simple modules. We will see that this can be determined by looking at
. Let
be the sequence where
for all
. We first show that it is sufficient to consider the case where
is a reduced pair for B. Note that in this case, we have
equal to the usual total order < on I.
Lemma 3.7.
[Citation22, Proposition 2.10] Suppose that B is a core block with a reduced pair for B. Let
. Then for
and k = 1, 2 we have
where
. We define
and
so that
for
and k = 1, 2; note that this abacus configuration does uniquely define both
and
and that
, so that if
is the
-equivalence class containing
then
is a reduced pair for
. Then
if and only if
.
Lemma 3.8.
Suppose that B is a core block and is a reduced pair for B. Let
. Suppose that
with
and let
be the bipartition whose Young diagram is formed by removing all removable i-nodes from
. Then unless
and
we have that
if and only if
.
Proof.
Since , each component of
has at most one removable i-node. If
has no removable i-nodes, the lemma is trivial. So suppose that
has one removable i-node. Then the i-residue sequence of
is either
,
or
. In the first two cases, this removable node is good, and so the lemma holds. In the third case, the first component of
has a removable i-node and the second has an addable i-node. Given
, this is only possible if
and
.
Now suppose that has two removable i-nodes so the i-residue sequence of
is
and the removable i-node in the first component is good. Removing this node gives a bipartition,
say, with
if and only if
. By the last paragraph, the one removable i-node in
is good, so that
if and only if
. □
Lemma 3.9.
Suppose that B is a core block with and
is a reduced pair for B. Let
. There is a bipartition
which satisfies the following conditions.
lies in a core block
and
is a reduced pair for
.
is essentially the same as
.
There exist
with
such that
For k = 1, 2,
for all
and
for all
.
for
.
Then if and only if
.
Before proving this result, we give an example of bipartitions and
which satisfy the lemma.
Example.
Below, we have and
.
Proof of Lemma 3.9.
Suppose there exist with j < i and
for k = 1, 2 while
for k = 1 or k = 2. Choose i minimal with this condition. Then following Lemma 3.8, we may remove all the removable i-nodes of
to obtain the Young diagram of a bipartition
with
if and only if
. Then the abacus configuration of
is formed from that of
by swapping runners i – 1 and i, so that
is formed from
by swapping the entries in the positions indexed by i – 1 and i; note that
so that
and
are essentially the same. We continue in this way until we reach a bipartition
where there do not exist i, j as above.
Now consider . Suppose there exist
with
and
for k = 1, 2 while
for k = 1 or k = 2. Choose
maximal with this property. Again following Lemma 3.8, we may remove all the removable
-nodes of
to obtain the Young diagram of a bipartition
with
if and only if
. Similarly to the case above,
is formed from
by swapping the entries in the positions indexed by
and
, and
so that
and
are essentially the same. We continue in this way until we reach a bipartition
where there do not exist
as above. Then
satisfies the conditions of the lemma and
if and only if
. □
Proposition 3.10.
Suppose that B is a core block and . Then
if and only if
.
Proof.
Suppose that is a reduced pair for
. By Lemma 3.7, we may assume that
. If
then the block
is simple and so
. So assume that
. By Lemma 3.9, we may assume that there exist
with
such that
For k = 1, 2,
for all
and
for all
.
for
.
We prove Proposition 3.10 by induction on . First suppose that
. If
then
, which we have assumed is not true. So suppose that
so that there are entries equal to both – and + in
with the + terms occurring before the – terms. The abacus configuration of
with respect to
looks like:
Then has only one removable node which is not good, hence
.
Now suppose that and Proposition 3.10 holds for any
in a core block with
. Then there exist
with
and
and
. By Lemma 3.8, we can remove the removable i + 1-node from
to obtain the Young diagram of a bipartition
with
if and only if
. Furthermore, we have
and
for
so that
if and only if
. Hence
where the middle step follows from the inductive hypothesis. □
In this section, we have shown that if B is a core block then we can index the Specht modules corresponding to bipartitions in B by a subset of the set of sequences with nB entries equal to – and pB entries equal to +, with the simple modules indexed by sequences
. The next step is to determine the entries in the block decomposition matrix. To do this, we begin by introducing the Fock space representation of
.
3.3 The Fock space representation
Let denote the quantized enveloping algebra
. This is a
-algebra with generators ei, fi for
and vh for
; the relations may be found in [Citation18]. Let
be the
-vector space with basis
. This becomes a
-module under the action described in [Citation18]; we call
the Fock space representation of
. The
-submodule
generated by
(where
is the unique bipartition of 0) is isomorphic to the irreducible highest weight module
for some dominant integral weight
of
. This module has a canonical basis (in the sense of Lusztig and Kashiwara) which is indexed by the elements of
; we write
for the canonical basis element indexed by
. We define the transition coefficients
so that they satisfy the equation
then
unless
. Ariki’s Theorem and its graded analogue, below, relate the transition coefficients to the graded decomposition numbers.
Theorem 3.11.
[Citation1, Citation5] Suppose that is defined over a field of characteristic 0. Suppose that
with
. Then
The transition coefficients give us the decomposition numbers when p = 0 and can be considered as a first approximation to the decomposition numbers when the field is arbitrary; as we shall see below, there are situations when this first approximation is correct. In order to compute the transition coefficients, we now consider the action of on a basis element
.
If d > 0 and , write
if
is formed from
by adding d nodes all of residue
. If
set
For d > 0 and , define
, the quantum divided power of
. Then if
,
Lemma 3.12.
[Citation22, Proposition 2.3] Suppose is such that if
then
. Let
and
. Suppose
where
for all
. Then
and if
then
.
The statement of Lemma 3.12 given in [Citation22] is only for the case when , but the proof for the slightly more general situation above is identical. Suppose
. If
, define
If e is finite and i > j, define
Suppose that B is a core block and that is a reduced pair for B. We say that
is flat if
or if e is finite and there exists
and
such that bm = b if
and
otherwise. Note that if e is finite then in this case,
Lemma 3.13.
Suppose that B is a core block and that is a reduced pair for B with
flat. Let
. Suppose that
with
Let , where
. Set
and define
Then
where
so that
Proof.
For and k = 1, 2, define
by
so that for k = 1, 2. We claim that if
then
Since is flat,
has exactly two addable
-nodes, one in the first component and one in the second, and no removable
-nodes. If we add an
-node to component k, for k = 1, 2, then the subsequent bipartition has exactly one addable
-node, in component k, and no removable
-nodes. Continuing in this way, we see that
Now suppose and consider
, where k = 1, 2. If
then
has exactly one addable
-node, which is in component k, and no removable
-nodes, and so
. Otherwise, as above,
where
is obtained by successively adding nodes to component
of
, for
. However, if
does not have any addable
-nodes so
. If
(resp.
) then
has an addable
-node on component 1 (resp. on component 2) and a removable
-node on component 2 (resp. on component 1) and so
(resp.
). So in both cases we have
The lemma follows. □
3.4 Decomposition matrices
Theorem 3.14.
Let B be a core block and suppose that is a reduced pair for B with
flat. Let
with
. Then
Proof.
We work by induction on . If
then B is simple and the result holds. So suppose
and the theorem holds for all core blocks
with
. Choose
such that there does not exist
with
. Let
be the bipartition defined by
Then for some core block
such that
is a reduced pair for
. Also
so
. Hence
satisfies the conditions of the inductive hypothesis, so by Theorem 3.11,
The bipartition with reduced pair
, together with the pair (i, j) described above, satisfies the conditions of Lemma 3.13. We define f as in that lemma and compute each term
that appears in the sum above, so that
Since for all
which appear in the sum, we have satisfied the conditions of Lemma 3.12 and so the theorem holds. □
Since there is always a reduced pair with
flat when
or n < e, this describes the block decomposition matrices for core blocks in these cases, recovering previous results; we note in particular the similarity between Theorem 3.14 and [Citation19, Theorem 3]. For the rest of this section, we assume that e is finite. Suppose that B and
are any two core blocks with
and
. Then we have a bijection
such that
and
are essentially the same for all
. If
then by Lemma 3.10, we have that
if and only if
.
Given two core blocks B and , we say that B and
are δ-equivalent if
and
and if the map
defined above preserves the block decomposition matrix of B, that is, if
with
then
. In fact, we will show that any two core blocks B and
with
and
are δ-equivalent. Theorem 3.14 shows that this also holds when
.
Lemma 3.15.
Let B be a core block and suppose that is a reduced pair for B. Suppose that
and
Define a map which sends
to the bipartition whose Young diagram is obtained by removing all possible
-nodes from
. Then
gives a bijection between B and a core block
such that if
then
.
Proof.
Suppose . From our assumptions on
has no addable
-nodes. If
then by Lemma 3.1,
for all
so that the number of removable
-nodes on both
and
is
Hence and
lie in the same block,
say. If
acts on the abacus configurations by swapping the runners i and i + 1 on each component, so that
and so by Corollary 3.2,
is a bijection between B and
. If
, then
is a reduced pair for
, where
It is straightforward to check that if then
so that again
and
is a bijection between B and
. □
Lemma 3.16.
Suppose that we have the conditions of Lemma 3.15. Keep the notation of that lemma so that . Suppose
is such that if
with
then
. Then
and B are δ-equivalent.
Proof.
Suppose that if then
has t removable
-nodes; equivalently if
then
has t addable
-nodes and no removable
-nodes. Let
. If
then
Let be such that
. Then
Hence by repeated application of Lemma 3.12, for
. Since
for all
, the lemma follows. □
Lemma 3.17.
Suppose that B is a core block with reduced pair . Then there exists a core block
with reduced pair
such that B and
are δ-equivalent and
is flat.
Proof.
Following Lemma 3.15, if there exists with
then we may apply the map
to B. If
then we may apply the map
to B. By repeatedly applying these maps, we obtain a core block
where
is a reduced pair for
and
is flat. By Theorem 3.14,
for all
with
. Hence by Lemma 3.16, we have that B and
are δ-equivalent. □
Corollary 3.18.
Suppose that B and are core blocks. If
and
then B and
are δ-equivalent.
Theorem 3.19.
Let B be a core block and suppose with
. Then
Proof.
Following Lemma 3.17, we know that B is δ-equivalent to a core block with reduced pair
where
is flat. The theorem then follows from Theorem 3.14. □
3.5 Summary and examples
We collect together the main results of this paper.
Theorem 3.20.
Let B be a core block and let . Let
and
.
We can identify a bipartition
with a sequence
which contains n entries equal to – and p entries equal to +. If
then
is formed by permuting these n + p entries. Hence
Suppose
. Then
if and only if
. Hence
We have
.
There exists
such that if
then if e is finite (resp.
) then
and
(resp.
and
).
5. Suppose that
with
. Then
Proof.
All these results appear in the previous section, apart from the formula for . To see that this holds, first suppose that
. Identify each sequence with n entries equal to – and p entries equal to + with a (n, p)-tableau by putting the entries corresponding to – in the top row and to + in the bottom row. The tableau is standard if and only if the sequence lies in
. Since the number of standard (n, p)-tableaux is given by the formula above, we are done. The case that n < p follows by symmetry. □
For in a core block B, let
denote the graded dimension of the homomorphism space between
and
. The next result appears in George Witty’s 2020 thesis [Citation27]. Although Witty only proves the case where
is a reduced pair for B, he conjectures that the result holds for an arbitrary reduced pair [Citation27, Conjecture 4.32]. It is possible that one could prove this conjecture using Scopes equivalence [Citation26].
Theorem 3.21.
[Citation27, Theorem 4.27] Suppose that B is a core block and that is a reduced pair for B. Suppose that
or that
. Let
. Then
If e = 2 or nB = pB then the formula above gives a lower bound for .
Witty’s theorem is an application of results proved in his thesis about homomorphisms between Specht modules. His proofs involve very intricate calculations within the cyclotomic KLR algebras.
It is likely that a formula similar to that of Theorem 3.20 (5) holds for the graded decomposition numbers for the type B Schur algebras when and it seems to us that an argument via induction on
, using the techniques developed in this paper, would prove the result. We thank the referee for asking this question, since our assumption when writing the paper was that our methods would not work for the Schur algebras.
We end this paper with some examples.
Example.
Suppose that e = 5 and . Let
and let B be the core block containing
. Then
is a reduced pair for B. By drawing the abacus configurations, we can see that nB = 3 and pB = 1. The four bipartitions in the block are given by the abacus configurations below.
Then the decomposition matrix for B is
Example.
Suppose that B is a core block with nB = 2 and pB = 3. Then and by removing the 0s from the sequences
we get Specht modules indexed by the 10 elements below. The block decomposition matrix for B is equal to
Note that this is the matrix that appears in [Citation19, Example 6].
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