Riemann-Roch for the flag variety is the Weyl Character formula!
More specifically, let LL be an ample line bundle on G/BG/B, corresponding to the weight lambdalambda. According to Borel-Weil-Bott, H0(G/B,L)H0(G/B,L) is VlambdaVlambda, the irreducible representation of GG with highest weight lambdalambda, and Hi(G/B,L)=0Hi(G/B,L)=0 for i>0i>0. So the holomorphic Euler characteristic of LL is mathrmdimVlambdamathrmdimVlambda.
As we will see, computing the holomorphic Euler characteristic of LL by Hirzebruch-Riemann-Roch gives the Weyl character formula for mathrmdimVlambdamathrmdimVlambda.
Notation:
GG is a simply-connected semi-simple algebraic group, BB a Borel and TT the maximal torus in BB. The corresponding Lie algebras are mathfrakgmathfrakg, mathfrakbmathfrakb, mathfraktmathfrakt. The Weyl group is WW, the length function on WW is ellell and the positive roots are Phi+Phi+. It will simplify many signs later to take BB to be a lower Borel, so the weights of TT acting on mathfrakbmathfrakb are Phi−Phi−.
We will need notations for the following objects:
rho=(1/2)sumalphainPhi+alpha.rho=(1/2)sumalphainPhi+alpha.
Delta=prodalphainPhi+alpha.Delta=prodalphainPhi+alpha.
delta=prodalphainPhi+(ealpha/2−e−alpha/2).delta=prodalphainPhi+(ealpha/2−e−alpha/2).
They respectively live in mathfrakt∗mathfrakt∗, in the polynomial ring mathbbC[mathfrakt∗]mathbbC[mathfrakt∗] and in the power series ring mathbbC[[mathfrakt∗]]mathbbC[[mathfrakt∗]].
Geometry of flag varieties
Every line bundle LL on G/BG/B can be made GG-equivariant in a unique way. Writing xx for the point B/BB/B, the Borel BB acts on the fiber LxLx by some character of TT. This is a bijection between line bundles on G/BG/B and characters of TT. Taking chern classes of line bundles gives classes in H2(G/B)H2(G/B). This extends to an isomorphism mathfrakt∗toH2(G/B,mathbbC)mathfrakt∗toH2(G/B,mathbbC) and a surjection mathbbC[[mathfrakt∗]]toH∗(G/B,mathbbC)mathbbC[[mathfrakt∗]]toH∗(G/B,mathbbC). We will often abuse notation by identifiying a power series in mathbbC[[mathfrakt∗]]mathbbC[[mathfrakt∗]] with its image in H∗(G/B)H∗(G/B).
We will need to know the Chern roots of the cotangent bundle to G/BG/B.
Again writing xx for the point B/BB/B, the Borel BB acts on the tangent space Tx(G/B)Tx(G/B) by the adjoint action of BB on mathfrakg/mathfrakbmathfrakg/mathfrakb. As a TT-representation, mathfrakg/mathfrakbmathfrakg/mathfrakb breaks into a sum of one dimensional representations, with characters the positive roots. We can order these summands to give a BB-equivariant filtration of mathfrakg/mathfrakbmathfrakg/mathfrakb whose quotients are the corresponding characters of BB. Translating this filtration around G/BG/B, we get a filtration on the tangent bundle whose associated graded is the direct sum of line bundles indexed by the negative roots. So the Chern roots of the tangent bundle are Phi+Phi+. (The signs in this paragraph would be reversed if BB were an upper Borel.)
The Weyl group WW acts on mathfrakt∗mathfrakt∗. This extends to an action of WW on H∗(G/B)H∗(G/B). The easiest way to see this is to use the diffeomorphism between G/BG/B and K/(KcapT)K/(KcapT), where KK is a maximal compact subgroup of GG; the Weyl group normalizes KK and TT so it gives an action on K/(KcapT)K/(KcapT).
We need the following formula, valid for any hinmathbbC[[mathfrakt∗]]hinmathbbC[[mathfrakt∗]]:
inth=mboxconstanttermofleft((sumwinW(−1)ell(w)w∗h)/Deltaright).quad(∗)inth=mboxconstanttermofleft((sumwinW(−1)ell(w)w∗h)/Deltaright).quad(∗)
Two comments: on the left hand side, we are considering hinH∗(G/B)hinH∗(G/B) and using the standard notation that intint means "discard all components not in top degree and integrate." On the right hand side, we are working in mathbbC[[mathfrakt∗]]mathbbC[[mathfrakt∗]], as DeltaDelta is a zero divisor in H∗(G/B)H∗(G/B).
Sketch of proof of (*)
: The action of ww is orientation reversing or preserving according to the sign of ell(w)ell(w). So inth=int(sumwinW(−1)ell(w)w∗h)/|W|inth=int(sumwinW(−1)ell(w)w∗h)/|W|. Since the power series sumwinW(−1)ell(w)w∗hsumwinW(−1)ell(w)w∗h is alternating, it is divisible by DeltaDelta and must be of the form Delta(k+(mboxhigherorderterms))Delta(k+(mboxhigherorderterms)) for some constant kk. The higher order terms, multiplied by DeltaDelta, all vanish in H∗(G/B)H∗(G/B), so we have inth=kintDelta/|W|inth=kintDelta/|W|. The right hand side of (∗)(∗) is just kk.
By the Chern root computation above, the top chern class of the tangent bundle is DeltaDelta. So intDeltaintDelta is the (topological) Euler characteristic of G/BG/B. The Bruhat decomposition of G/BG/B has one even-dimensional cell for every element of WW, and no odd cells, so intDelta=|W|intDelta=|W| and we have proved formula (∗)(∗).
The computation
We now have all the ingredients. Consider an ample line bundle LL on G/BG/B, corresponding to the weight lambdalambda of TT. The Chern character is elambdaelambda. HRR tells us that the holomorphic Euler characteristic of LL is
intelambdaprodalphainPhi+fracalpha1−e−alpha.intelambdaprodalphainPhi+fracalpha1−e−alpha.
Elementary manipulations show that this is
intfracelambda+rhoDeltadelta.
Applying (∗), and noticing that Delta/delta is fixed by W, this is
mboxConstanttermofleft(frac1DeltafracDeltadeltasumwinW(−1)ell(w)w∗elambda+rhoright)=
mboxConstanttermofleft(fracsumwinW(−1)ell(w)ew(lambda+rho)deltaright).
Let slambda be the character of the G-irrep with highest weight lambda. By the Weyl character formula, the term in parentheses is slambda as an element of mathbbC[[mathfrakt∗]]. More precisely, a character is a function on G. Restrict to T, and pull back by the exponential to get an analytic function on mathfrakt. The power series of this function is the expression in parentheses. Taking the constant term means evaluating this character at the origin, so we get dimVlambda, as desired.
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