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Phan systems study group II

October 17, 2009

Here is the next instalment in our Phan systems study group which was held yesterday.  I forgot to bring my laptop  so I took notes by hand. Blogging the study group has already had one benefit as Gordon is now attending. Having been initially put off by the title he has realised that we are actually doing lots of stuff that he is interested in learning.

John began the discussion by working through the geometry of the symplectic polar space of rank 2.  Let J=\left(\begin{array}{cc} 0_2 &I_2\\ -I_2& 0_2\end{array}\right) and let \beta be the bilinear form on V=K^4, for some field K, given by \beta(v,w)=vJw^T. Note we are using row vectors, and if v=(v_1,v_2,v_3,v_4), w=(w_1,w_2,w_3,w_4) we have \beta(v,w)=v_1w_3-v_3w_1+v_2w_4-v_4w_2. Then \beta is an alternating form, that is \beta(v,w)=-\beta(w,v) and \beta(v,v)=0 for all v,w\in V. The totally isotropic subspaces of V, are those subspaces W for which the restriction \beta_{\mid W} is zero, that is, \beta(v,w)=0 for all v,w\in W. We can then form a point-line geometry whose points are the totally isotropic 1-spaces of V and the lines are the totally isotropic 2-spaces of V.  Note that all 1-spaces of V are totally isotropic, but V does not contain any totally isotropic 3-spaces. A hyperbolic pair is a pair of vectors (e,f) such that \beta(e,f)=1.

Let e_1=(1,0,0,0), e_2=(0,1,0,0), f_1=(0,0,1,0) and f_2=(0,0,0,1). Then (e_1,f_1) and (e_2,f_2) are hyperbolic pairs, while \langle e_1,e_2\rangle,\langle f_1,f_2\rangle,\langle e_1,f_2\rangle and \langle e_2,f_1\rangle are totally isotropic. We have V=\langle e_1,f_1\rangle\perp\langle e_2,f_2\rangle, that is, V can be written as an orthogonal direct sum of two hyperbolic lines. One of the chambers in this geometry is \langle e_1\rangle\subset \langle e_1,e_2\rangle.

The point-line geometry we have just defined is an example of a generalised quadrangle. That is, it satisfies the following two axioms:

  • Any two points lie on at most one line.
  • Given  a line \ell and a point p not incident with \ell, there is a unique point q on \ell which is incident with p.

Such a geometry is often referred to as a geometry of type B_2  and is denoted by the diagram b2

In general, we can define an alternating bilinear form on K^{2n} by using the matrix \left(\begin{array}{cc} 0_n& I_n\\-I_n&0_n\end{array}\right). The set of all totally isotropic subspaces of K^{2n} forms the symplectic polar space of rank n.  The maximal totally isotropic subspaces have dimension n and K^{2n} can be written as an orthogonal direct sum of n hyperbolic lines. This geometry has the diagram B_n:bnwhere the nodes of the diagram represent from left to right the totally isotropic 1-spaces, totally isotropic 2-spaces through to the totally isotropic n-spaces. Recall that the edge between nodes i and j represents the geometry formed by the residue of a flag containing elements of all types except i and j. So for example, if we take a flag F=U_3\subset U_4\subset \cdots\subset U_n where each U_i is a totally isotropic i-space then the residue of F is the set of all totally isotropic 1-spaces and 2-spaces incident with F. Since U_3 is totally isotropic, this is all the 1-spaces and 2-spaces contained in U_3 and hence is a projective plane. This is why a single edge is placed between the nodes representing 1-spaces and 2-spaces. At the other end of the diagram, if we take a flag F'=U_1\subset U_2\subset \cdots \subset U_{n-2} with each U_i totally isotropic of dimension i, then the residue of F' is the set of all totally isotropic n-spaces and (n-1)-spaces containing U_{n-2}. Since U_{n-2}^\perp=\{v\in V\mid \beta(v,u)=0 \text{ for all } u\in U_{n-2}\} has dimension 2n-(n-2)=n+2 and U_{n-2}^\perp/U_{n-2} has dimension 4 and is isomorphic to our original symplectic polar space of rank 2, we have a double bond between the last two nodes in the diagram.

John mentioned that whereas a geometry with the same diagram as a projective space is indeed a projective space, a geometry with the diagram B_n is either a symplectic polar space or the Neumaier geometry. Hopefully we learn more about this is subsequent weeks.

[added19/10/09: John has pointed out that I have misunderstood him here. The B_2 diagram is used to denote any generalised quadrangle, and a geometry with diagram B_n is either a polar space or the Neumaier geometry. There are polar spaces other than the symplectic one with diagram B_n. This is something we should be covering later.]

Now onto the related groups.  The symplectic group \mathrm{Sp}(4,K) is the group of all linear transformations of V=K^4 which preserves \beta. This is all matrices A\in \mathrm{GL}(4,K) such that AJA^T=J. Writing A as \displaystyle{ \left(\begin{array}{cc} A_1&A_2\\A_3&A_4\end{array}\right)} with each A_i a 2\times 2 matrix we see that we must have A_3^TA_1-A_1^TA_3=0_2= A_4^TA_2-A_2^TA_4 and A_4A_1^T-A_3A_2^T=I_2.

A Borel subgroup B is the stabiliser of a chamber. Using our chamber \langle e_1\rangle\subset\langle e_1,e_2\rangle we have that B is all matrices in \mathrm{Sp}(4,K) with A_1 lower triangular, A_2=0 and A_4=A_1^{-T}, the inverse-transpose of A_1.

A frame is \mathcal{F}=\{\langle e_1\rangle,\langle e_2\rangle,\langle f_1\rangle,\langle f_2\rangle\}. Then N is the setwise stabiliser of \mathcal{F} and so is a group of monomial matrices. The group N must preserve the partition \{\{\langle e_1\rangle,\langle f_2\rangle,\{\langle e_2\rangle,\langle f_2\rangle\} and so N^{\mathcal{F}}=C_2\mathrm{ wr } C_2. The base group of this wreath product is generated by the matrices

\displaystyle{\left(\begin{array}{cccc} 0&0&1&0\\ 0&1&0&0\\1&0&0&0\\ 0&0&0&1\end{array}\right), \left(\begin{array}{cccc} 1&0&0&0\\ 0&0&0&1\\ 0&0&1&0\\ 0&1&0&0\end{array}\right)}.

These matrices interchange \langle e_1\rangle and \langle f_1\rangle, and \langle e_2\rangle and \langle f_2\rangle  respectively. The top group is given by all matrices

\displaystyle{\left(\begin{array}{cc}P&0_2\\ 0_2&P^{-T}\end{array}\right)}

where P is a 2\times 2 permutation matrix. The kernel T of the action of N on \mathcal{F} is all diagonal matrices of the form

\displaystyle{\left(\begin{array}{cccc} \lambda_1&0&0&0\\ 0&\lambda_2&0&0\\ 0&0&\lambda_1^{-1}&0\\ 0&0&0&\lambda_2^{-1}\end{array}\right)}.

So the Weyl group of \mathrm{Sp}(4,K) is N/T\cong C_2\mathrm{ wr }C_2.

This can all be generalised to \mathrm{Sp}(2n,K) where we have that the Weyl group is N/T\cong C_2\mathrm{ wr }S_n, the Coxeter group of type B_n. This is also thought of as the group of signed permutations of the set \{ \pm1,\pm2,\ldots,\pm n\}.

To finish off, Alice spoke briefly about the apartments in this setting. Whereas I had initially interpreted her as saying that in the A_n case an apartment was just the same as a frame, it is actually more subtle than that. In fact an apartment is the set of all flags which you can get from the elements of a frame. Thus in the case of \mathrm{Sp}(4,K), the apartment corresponding to \mathcal{F}=\{\langle e_1\rangle,\langle e_2\rangle,\langle f_1\rangle,\langle f_2\rangle\} is the set of all flags whose elements are in \mathcal{F}.This can be represented in the following picture where the vertices are 1-spaces and the edges are 2-spaces. Each vertex-edge incident pair is a rank 2 flag. B2apartment

The chamber system of this apartment is then given below. I have only labelled three of the nodes.B2chamberThe red edges represent that the 1-spaces of the chamber are different while the green edges represent that the 2-spaces of the chamber are different. You can define reflections s_1,s_2 of the apartment (the square picture above) such that s_1 is the reflection about the vertical axis of the square and s_2 is the reflection about the diagonal axis from the top left corner.  Then starting in the bottom left hand corner of the hexagon and working clockwise we get the sequence 1,s_2,s_2s_1,s_2s_1s_2,s_2s_1s_2s_1 which maps the bottom left chamber around to the top right hand corner chamber. Working anticlockwise we get 1,s_1,s_1s_2,s_1s_2s_1,s_1s_2s_1s_2 around to the top right hand corner. Since the diagram should commute we get s_2s_1s_2s_1=s_1s_2s_1s_2 and so (s_1s_2)^4=1, that is we get the Coxeter relation for groups of type B_2.

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2 Comments leave one →
  1. Alice Devillers permalink
    October 20, 2009 10:15 am

    One rectification:
    the other polar spaces (quadratic, hermitian) are also of type B_n.

  2. October 20, 2009 10:41 am

    Neumaier’s A_7-geoemtry is an example of a geometry which has the B_3 (or more accurately, the C_3) diagram. Take the set {1,2,3,4,5,6,7} and consider the 30 Fano planes we can make out of this set (take an orbit of one under S_7). These projective planes break into two orbits of size 15 each under A_7). Now we define a geometry by…

    POINTS: {1,2,3,4,5,6,7}
    LINES: 3-element subsets of {1,2,3,4,5,6,7}
    PLANES: One orbit of size 15 on Fano planes (under A_7).

    Incidence is simply induced by set membership, and we obtain a rank 3 geometry with the residues giving us a C_3 geometry. To see this, if we fix one PLANE, that is, a Fano plane, the POINTS and LINES contained in it are the points and lines of the Fano plane. So this residue is clearly a projective plane. If we fix a POINT, we have 15 LINES containing it and every PLANE containing it. So we obtain a rank 2 residue with 15 elements of one type and 15 elements of the other type. We in fact obtain the smallest thick generalised quadrangle W(2).

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