Standard tableaux for type C in terms of boxes

Arun Ram
Department of Mathematics and Statistics
University of Melbourne
Parkville, VIC 3010 Australia
aram@unimelb.edu.au

Last update: 15 November 2012

Standard tableaux for type C in terms of boxes

8.1. The root system

Let {ε1,,εn} be an orthonormal basis of 𝔥*=n and view elements γ=iγiεi of n as sequences

γ= ( γ-n,, γ-1;γ1 ,,γn ) ,such that γ-i= -γi. (8.2)

The root system of type Cn is given by the sets

R = { ±2εi, ±(εj±εi) 1i, jn } and R+ = { 2εi,εj ±εi 1i<jn } . (8.3)

The simple roots are given by a1=2ε1, αi=εi-εi-1, 1in. The Weyl group W=WCn is the hyperoctahedral group of permutations of -n,,-1,1,,n such that w(-i)=-w(i). This groups acts on the εi by the rule wεi=εw(i), with the convention that ε-i=-εi.

For this type C case there is a nice trick. View the root system as

R = { ±(εj±εi) 1<j, i,j {±1,,±n} } and R+ = { εj-εi i<j,i,j {±1,,±n} } , (8.4)

with the convention that ε-i=-εi. In this notation εi-ε-i=2εi and ε-i- ε-j= εj-εi. This way the type C root system “looks like” a type A root system and many computations can be done in the same way as in type A.

8.5. Rearranging γ

We analyze the structure of the sets (γ,J) as considered in (4.3). This corresponds to when the q in the affine Hecke algebra is not a root of unity. The analysis in this case is analogous to the method that was used in Section 5.11 to create books of placed configurations in the type A case.

Let γn. Apply an element of the Weyl group to γ to “arrange” the entries of γ so that, for each i{1,,n},

γi [z+12,z], for somez.

Then

γ-i=-γi [z,z+12] .for somez .

As in the type A case, the sets Z(γ) and P(γ) can be partitioned according to the cosets of the elements of γ and it is sufficient to consider each -coset separately and then assemble the results in “books of pages.” There are three cases to consider:

Case β. The -coset β+, β(1/2,1). Then

γ= ( -β-zn-β -z2-β-z1; β+z1β+z2 β+zn ) ,z1,

Case 1/2. The -coset 1/2+. Then

γ = ( -1/2-zn -1/2-z2- 1/2-z1; 1/2+z11/2 +z21/2 +zn ) ,zi 0,

Case 0. The -coset . Then

γ= ( -zn-z2 -z1;z1 z2zn ) ,zi0.

It is notationally convenient to let z-i=-zi.

8.6. Boxes and standard tableaux

Let us assume that the entries of γ all lie in a single -coset and describe the resulting standard tableaux. The general case is obtained by creating books of pages of standard tableaux where the pages correspond to the different -cosets of entries in γ.

The placed configuration of boxes is determined as follows.

8.7. Case β,β(1/2,1)

Assume that γ𝔥* is of the form

γ= ( -β-zn-β- z2-β-z1; β+z1β+z2 β+zn ) ,zi.

Place boxes on two pages of infinite graph paper. These pages are numbered β and -β and each page has the diagonals numbered consecutively with the elements of , from bottom left to top right. View these two pages, page β and page -β, as “linked.” For each 1in place boxi on diagonal zi of page β and box-i on diagonal -zi of page -β. The boxes on each diagonal are arranged in increasing order from top left to bottom right. The placement of boxes on page -β is a 180 rotation of the placement of the boxes on page β.

Using the notation for the root system of type Cn in (8.4)

P(γ) = { εj-εi j>iand boxiandboxj are in adjacent diagonals } and Z(γ) = { εj-εi j>iand boxiandboxj are in the same diagonal } .

Note that ε-i-ε-jZ(γ) if and only if εj-εiZβ(γ), and similarly ε-i-ε-jPβ(γ) if and only if εj-εiPβ(γ). If JP(γ) arrange the boxes on adjacent diagonals according to the rules

  1. if εj-εiJ place boxj northwest of boxi, and
  2. if εj-εiP(γ)\J place boxj southeast of boxi.

A standard tableau is a negative rotationally symmetric filling p of the 2n boxes with -n,,-1,1,,n such that

  1. p(boxi)<p(boxj) if j>i and boxj and boxi are in the same diagonal,
  2. p(boxi)>p(boxj) if j>i, boxi and boxj are in adjacent diagonals and boxj is northwest of boxi,
  3. p(boxi)<p(boxj) if j>i, boxi and boxj are in adjacent diagonals and boxj is southeast of boxi.

The negative rotational symmetry means that the filling of the boxes on page -β is the same as the filling on page β except rotated by 180 and with all entries in the boxes multiplied by -1.

Example. Suppose β(1/2,1), and

γ = (-β;β) + ( -2,-2,-2, -1,-1,-1, 0,0,0, 1,1,1; -1,-1,-1, 0,0,0, 1,1,1, 2,2,2 ) = ( -β-2, -β-2, -β-2, -β-1, -β-1, -β-1, -β,-β,-β, -β+1, -β+1, -β+1; β-1, β-1, β-1, β,β,β, β+1, β+1, β+1, β+2, β+2, β+2

and

J = { ε4-ε1, ε-1-ε-4, ε4-ε2, ε-2-ε-1, ε4-ε3, ε-3-ε-4, ε5-ε2, ε-2-ε-5, ε5-ε3, ε-3-ε-5, ε7-ε5, ε-5-ε-7, ε7-ε6, ε-6-ε-7, ε8-ε6, ε-6-ε-8, ε10-ε9, ε-9-ε-10, ε10-ε8, ε-8-ε-10, ε10-ε7, ε-7-ε-10, ε11-ε9, ε-9-ε-11, ε11-ε8, ε-8-ε-11, ε11-ε7, ε-7-ε-11, ε12-ε9 ε-9-ε-12 } .

The placed configuration of boxes corresponding to (γ,J) is

-1 0 1 2 -2 -1 1 2 0 1 0 1 -1 0 -1 0 -2 -1 1 2 -2 -1 0 1 Page-β Pageβ -9 -6 -3 10 -12 -8 -2 11 -5 -1 4 7 -7 -4 1 5 -11 2 8 12 -10 3 6 9 Page-β Pageβ contents of boxes numbering of boxes

and a sample negative rotationally symmetric standard tableau is

-4 -3 5 -11 -6 -1 7 -10 8 9 -9 -8 2 12 -9 -8 10 -7 1 6 11 -5 3 4 Page-β Pageβ a standard tableau

8.8. Case 1/2

Assume that γ𝔥* is of the form

γ = ( -1/2-zn -1/2-z2-1/ 2-z1; 1/2+z11/2+ z21/2+ zn ) ,zi 0.

Place boxes on a page of infinite graph paper which has its diagonals numbered consecutively with the elements of 1/2+, from bottom left to top right. This page has page number 1/2. For each i{±1,,±n} place boxi on diagonal 1/2+zi and box-i on diagonal -1/2-zi. The boxes on each diagonal are arranged in increasing order from top left to bottom right and the placement of boxes is negative rotationally symmetric in the sense that a 180 rotation takes boxi to box-i.

Using the root system notation in (8.4),

P(γ) = { εj-εi j>iand boxiandboxj are in adjacent diagonals } and Z(γ) = { εj-εi j>iand boxiandboxj are in the same diagonal } .

Note that it is the formulation of the root system of type Cn in (8.4) which makes the description of P(γ) and Z(γ) nice in this case. If JP(γ) arrange the boxes on adjacent diagonals according to the rules

  1. if εj-εiJ place boxj northwest of boxi, and
  2. if εj-εiP(γ)\J place boxj southeast of boxi.

A standard tableau is a negative rotationally symmetric filling p of the 2n boxes with -n,,-1,1,,n such that

  1. p(boxi)<p(boxj) if j>i and boxj and boxi are in the same diagonal,
  2. p(boxi)>p(boxj) if j>i, boxi and boxj are in adjacent diagonals and boxj is northwest of boxi,
  3. p(boxi)<p(boxj) if j>i, boxi and boxj are in adjacent diagonals and boxj is southeast of boxi.

The negative rotational symmetry means that the filling of the boxes is the same if each entry is multiplied by -1 and the configuration is rotated by 180.

Example. Suppose

γ= ( -72, -52, -52, -32, -32, -32, -32, -12, -12, -12, -12, 12, 12, 12, 12, 32, 32, 32, 32, 52, 52, 72 )

and

J = { ε11-ε10, ε-10-ε-11, ε10-ε8, ε-8-ε-10, ε9-ε7, ε-7-ε-9, ε9-ε8, ε-8-ε-9, ε7-ε3, ε-3-ε-7, ε7-ε4, ε-4-ε-7, ε6-ε2, ε-2-ε-6, ε6-ε3, ε-3-ε-6, ε6-ε4, ε-4-ε-6, ε5-ε4, ε-4-ε-5, ε5-ε3, ε-3-ε-5, ε5-ε2, ε-2-ε-5, ε2-ε-1, ε3-ε-1, ε4-ε-1, ε1-ε-1 } . = { ε11-ε10, ε10-ε8, ε9-ε7, ε9-ε8, ε7-ε3, ε7-ε4, ε6-ε2, ε6-ε3, ε6-ε4, ε5-ε4, ε5-ε3, ε5-ε2, ε2+ε1, ε3+ε1, ε4+ε1, 2ε1 } .

The placed configuration of boxes corresponding to (γ,J) is as given below:

12 -32 -12 32 -52 -12 32 52 -72 -32 -12 12 32 72 -52 -32 12 52 -32 12 32 -12 1 -8 -4 5 -10 -3 6 9 -11 -7 -2 2 7 11 -9 -6 3 10 -5 4 8 -1 -11 -7 -6 -10 -5 -4 -9 -8 -3 -2 -1 1 2 3 8 9 4 5 10 6 7 11 Page12 Page12 Page12 contents of boxes numbering of boxes a standard tableau

8.9. Case 0

Assume that γ𝔥* is of the form

γ= ( -zn-z2 -z1; z1z2 zn ) ,zi0.

Place boxes on a page of infinite graph paper which has its diagonals numbered consecutively with the elements of , from bottom left to top right. This page has page number 0. For each i{±1,,±n} place boxi on diagonal zi and box-i on diagonal -zi. The boxes on each diagonal are arranged in increasing order from top left to bottom right and the placement of boxes is negative rotationally symmetric in the sense that a 180 rotation takes boxi to box-i.

Using the root system notation in (8.4),

P(γ) = { εj-εi j>iand boxiandboxj are in adjacent diagonals } and Z(γ) = { εj-εi j>iand boxiandboxj are in the same diagonal } .

If JP(γ) arrange the boxes on adjacent diagonals according to the rules

  1. if εj-εiJ place boxj northwest of boxi, and
  2. if εj-εiP(γ)\J place boxj southeast of boxi.

A standard tableau is a negative rotationally symmetric filling p of the 2n boxes with -n,,-1,1,,n such that

  1. p(boxi)<p(boxj) if j>i and boxj and boxi are in the same diagonal,
  2. p(boxi)>p(boxj) if j>i, boxi and boxj are in adjacent diagonals and boxj is northwest of boxi,
  3. p(boxi)<p(boxj) if j>i, boxi and boxj are in adjacent diagonals and boxj is southeast of boxi.

The negative rotational symmetry means that the filling of the boxes is the same if each entry is multiplied by -1 and the configuration is rotated by 180.

Example. Suppose γ= ( -2,-1,-1,-1, 0,0,0,0,0,0, 1,1,1,2 ) and

J = { ε4-ε1, ε-1-ε-4, ε4-ε2, ε-2-ε-4, ε4-ε3, ε-3-ε-2, ε5-ε1, ε-1-ε-5, ε5-ε2, ε-2-ε-5, ε5-ε3, ε-3-ε-5, ε6-ε1, ε-1-ε-6, ε6-ε2, ε-2-ε-6, ε6-ε3, ε-3-ε-6, ε7-ε6, ε-6-ε-7, ε6-ε-1, ε1-ε-6, ε5-ε-1, ε1-ε-5, ε4-ε-1, ε1-ε-4, ε5-ε-2, ε2-ε-5, ε4-ε-2, ε2-ε-4 } . = { ε4-ε1, ε4-ε2, ε4-ε3, ε5-ε1, ε5-ε2, ε5-ε3, ε6-ε1, ε6-ε2, ε6-ε3, ε7-ε6, ε6+ε1, ε5+ε1, ε5+ε2, ε4+ε1, ε4+ε2 } .

The placed configuration of boxes corresponding to (γ,J) is as given below:

0 1 1 2 0 1 0 0 -1 0 -2 -1 -1 0 -3 4 5 7 -2 6 -1 1 -6 2 -7 -5 -4 3 -7 -6 -5 -4 -3 -2 -1 1 2 3 4 5 6 7 Page 0 Page 0 Page 0 contents of boxes numbering of boxes a standard tableau

8.10.

A posteriori the analysis of the three cases β, 1/2, and 0, it becomes evident that the trick of using the formulation of the root system of type Cn in (8.4) provides a completely uniform description of the configurations of boxes and standard tableaux corresponding to type Cn local regions. All three cases give negative rotationally invariant tableaux. We could not ask for nature to work out more perfectly.

Notes and References

This is an excerpt of the paper entitled Affine Hecke algebras and generalized standard Young tableaux written by Arun Ram in 2002, published in the Academic Press Journal of Algebra 260 (2003) 367-415. The paper was dedicated to Robert Steinberg.

Research supported in part by the National Science Foundation (DMS-0097977) and the National Security Agency (MDA904-01-1-0032).

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