## The honeycomb model of $GL_n({\mathbb C})$ tensor products II: Puzzles determine facets of the Littlewood-Richardson cone

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- by Allen Knutson, Terence Tao and Christopher Woodward
- J. Amer. Math. Soc.
**17**(2004), 19-48 - DOI: https://doi.org/10.1090/S0894-0347-03-00441-7
- Published electronically: October 14, 2003
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Part I: J. Amer. Math. Soc.

**12**(1999), 1055-1090

## Abstract:

The set of possible spectra $(\lambda ,\mu ,\nu )$ of zero-sum triples of Hermitian matrices forms a polyhedral cone, whose facets have been already studied by Knutson and Tao, Helmke and Rosenthal, Totaro, and Belkale in terms of Schubert calculus on Grassmannians. We give a complete determination of these facets; there is one for each triple of Grassmannian Schubert cycles intersecting in a unique point. In particular, the list of inequalities determined by Belkale to be sufficient is in fact minimal. We introduce*puzzles*, which are new combinatorial gadgets to compute Grassmannian Schubert calculus, and seem to have much interest in their own right. As the proofs herein indicate, the Hermitian sum problem is very naturally studied using puzzles directly, and their connection to Schubert calculus is quite incidental to our approach. In particular, we get new, puzzle-theoretic, proofs of the results of Horn, Klyachko, Helmke and Rosenthal, Totaro, and Belkale. Along the way we give a characterization of โrigidโ puzzles, which we use to prove a conjecture of W. Fulton: โif for a triple of dominant weights $\lambda ,\mu ,\nu$ of $GL_n({\mathbb C})$ the irreducible representation $V_\nu$ appears exactly once in $V_\lambda \otimes V_\mu$, then for all $N\in {\mathbb N}$, $V_{N\lambda }$ appears exactly once in $V_{N\lambda }\otimes V_{N\mu }$.โ

## References

- Prakash Belkale,
*Local systems on $\Bbb P^1-S$ for $S$ a finite set*, Compositio Math.**129**(2001), no.ย 1, 67โ86. MR**1856023**, DOI 10.1023/A:1013195625868
[DW1]DW1 H. Derksen, J. Weyman, On the $\sigma$-stable decomposition of quiver representations, preprint available at http://www.math.lsa.umich.edu/~hderksen/preprint.html.
[DW2]DW2 H. Derksen, J. Weyman, On the Littlewood-Richardson polynomials, preprint available at http://www.math.lsa.umich.edu/~hderksen/preprint.html.
- William Fulton,
*Eigenvalues, invariant factors, highest weights, and Schubert calculus*, Bull. Amer. Math. Soc. (N.S.)**37**(2000), no.ย 3, 209โ249. MR**1754641**, DOI 10.1090/S0273-0979-00-00865-X - William Fulton,
*Young tableaux*, London Mathematical Society Student Texts, vol. 35, Cambridge University Press, Cambridge, 1997. With applications to representation theory and geometry. MR**1464693** - Oleg Gleizer and Alexander Postnikov,
*Littlewood-Richardson coefficients via Yang-Baxter equation*, Internat. Math. Res. Notices**14**(2000), 741โ774. MR**1776619**, DOI 10.1155/S1073792800000416 - Uwe Helmke and Joachim Rosenthal,
*Eigenvalue inequalities and Schubert calculus*, Math. Nachr.**171**(1995), 207โ225. MR**1316359**, DOI 10.1002/mana.19951710113 - Alfred Horn,
*Eigenvalues of sums of Hermitian matrices*, Pacific J. Math.**12**(1962), 225โ241. MR**140521**, DOI 10.2140/pjm.1962.12.225 - Allen Knutson and Terence Tao,
*The honeycomb model of $\textrm {GL}_n(\textbf {C})$ tensor products. I. Proof of the saturation conjecture*, J. Amer. Math. Soc.**12**(1999), no.ย 4, 1055โ1090. MR**1671451**, DOI 10.1090/S0894-0347-99-00299-4
[Kl]Kly A. A. Klyachko, Stable vector bundles and Hermitian operators, IGM, University of Marne-la-Vallee preprint (1994).
- Allen Knutson,
*The symplectic and algebraic geometry of Hornโs problem*, Linear Algebra Appl.**319**(2000), no.ย 1-3, 61โ81. Special Issue: Workshop on Geometric and Combinatorial Methods in the Hermitian Sum Spectral Problem (Coimbra, 1999). MR**1799624**, DOI 10.1016/S0024-3795(00)00220-2
[KT1]Puz1 A. Knutson, T. Tao, Puzzles and (equivariant) cohomology of Grassmannians, in preparation.
[KT2]Puz2 A. Knutson, T. Tao, Puzzles, Littlewood-Richardson rings, and the legend of Procrustes, in preparation.
- D. Mumford, J. Fogarty, and F. Kirwan,
*Geometric invariant theory*, 3rd ed., Ergebnisse der Mathematik und ihrer Grenzgebiete (2) [Results in Mathematics and Related Areas (2)], vol. 34, Springer-Verlag, Berlin, 1994. MR**1304906**, DOI 10.1007/978-3-642-57916-5 - Burt Totaro,
*Tensor products of semistables are semistable*, Geometry and analysis on complex manifolds, World Sci. Publ., River Edge, NJ, 1994, pp.ย 242โ250. MR**1463972**

## Bibliographic Information

**Allen Knutson**- Affiliation: Mathematics Department, UC Berkeley, Berkeley, California 94720
- Email: allenk@math.berkeley.edu
**Terence Tao**- Affiliation: Mathematics Department, UCLA, Los Angeles, California 90095-1555
- MR Author ID: 361755
- ORCID: 0000-0002-0140-7641
- Email: tao@math.ucla.edu
**Christopher Woodward**- Affiliation: Mathematics Department, Rutgers University, New Brunswick, New Jersey 08854-8019
- MR Author ID: 603893
- Email: ctw@math.rutgers.edu
- Received by editor(s): July 2, 2001
- Published electronically: October 14, 2003
- Additional Notes: The first author was supported by an NSF Postdoctoral Fellowship, an NSF grant, and the Clay Mathematics Institute

The second author was supported by the Clay Mathematics Institute, and grants from the Sloan and Packard Foundations

The third author was partially supported by an NSF Postdoctoral Fellowship, and NSF Grant 9971357 - © Copyright 2003 American Mathematical Society
- Journal: J. Amer. Math. Soc.
**17**(2004), 19-48 - MSC (2000): Primary 14N15; Secondary 15A42, 52B12, 05E10
- DOI: https://doi.org/10.1090/S0894-0347-03-00441-7
- MathSciNet review: 2015329