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Reductive quotients of klt singularities. (English) Zbl 07895069

Summary: We prove that the quotient of a klt type singularity by a reductive group is of klt type in characteristic 0. In particular, given a klt variety \(X\) endowed with the action of a reductive group \(G\) and admitting a quasi-projective good quotient \(X\rightarrow X/\!/G\), we can find a boundary \(B\) on \(X/\!/G\) so that the pair \((X/\!/G,B)\) is klt. This applies for example to GIT-quotients of klt varieties. Our main result has consequences for complex spaces obtained as quotients of Hamiltonian Kähler \(G\)-manifolds, for collapsings of homogeneous vector bundles as introduced by Kempf, and for good moduli spaces of smooth Artin stacks. In particular, it implies that the good moduli space parametrizing \(n\)-dimensional K-polystable smooth Fano varieties of volume \(v\) has klt type singularities. As a corresponding result regarding global geometry, we show that quotients of Mori Dream Spaces with klt Cox rings are Mori Dream Spaces with klt Cox ring. This in turn applies to show that projective GIT-quotients of varieties of Fano type are of Fano type; in particular, projective moduli spaces of semistable quiver representations are of Fano type.

MSC:

14B05 Singularities in algebraic geometry
14E30 Minimal model program (Mori theory, extremal rays)
14L24 Geometric invariant theory
14M25 Toric varieties, Newton polyhedra, Okounkov bodies
14A20 Generalizations (algebraic spaces, stacks)
53D20 Momentum maps; symplectic reduction

References:

[1] Alper, J., Good moduli spaces for Artin stacks, Ann. Inst. Fourier (Grenoble), 63, 6, 2349-2402, 2013 · Zbl 1314.14095 · doi:10.5802/aif.2833
[2] Alper, J.; Hall, J.; Rydh, D., A Luna étale slice theorem for algebraic stacks, Ann. Math. (2), 191, 3, 675-738, 2020 · Zbl 1461.14017 · doi:10.4007/annals.2020.191.3.1
[3] Altmann, K.; Hausen, J., Polyhedral divisors and algebraic torus actions, Math. Ann., 334, 3, 557-607, 2006 · Zbl 1193.14060 · doi:10.1007/s00208-005-0705-8
[4] Altmann, K.; Ilten, N. O.; Petersen, L.; Süß, H.; Vollmert, R., The geometry of \(T\)-varieties, Contributions to Algebraic Geometry, 17-69, 2012, Zürich: Eur. Math. Soc., Zürich · Zbl 1316.14001 · doi:10.4171/114-1/2
[5] Ambro, F., Shokurov’s boundary property, J. Differ. Geom., 67, 2, 229-255, 2004 · Zbl 1097.14029 · doi:10.4310/jdg/1102536201
[6] Arbarello, E.; Cornalba, M.; Griffiths, P. A.; Harris, J., Geometry of Algebraic Curves. Vol. I, 1985, New York: Springer, New York · Zbl 0559.14017 · doi:10.1007/978-1-4757-5323-3
[7] Arzhantsev, I.; Derenthal, U.; Hausen, J.; Laface, A., Cox Rings, 2015, Cambridge: Cambridge University Press, Cambridge · Zbl 1360.14001
[8] Arzhantsev, I.; Braun, L.; Hausen, J.; Wrobel, M., Log terminal singularities, platonic tuples and iteration of Cox rings, Eur. J. Math., 4, 1, 242-312, 2018 · Zbl 1408.14148 · doi:10.1007/s40879-017-0179-8
[9] Ascher, K.; DeVleming, K.; Liu, Y., K-stability and birational models of moduli of quartic K3 surfaces, Invent. Math., 232, 2, 471-552, 2023 · Zbl 1535.14087 · doi:10.1007/s00222-022-01170-5
[10] Bäker, H., Good quotients of Mori dream spaces, Proc. Am. Math. Soc., 139, 9, 3135-3139, 2011 · Zbl 1230.14066 · doi:10.1090/S0002-9939-2011-10742-1
[11] Bingener, J.; Flenner, H., Variation of the divisor class group, J. Reine Angew. Math., 351, 20-41, 1984 · Zbl 0542.14003 · doi:10.1515/crll.1984.351.20
[12] Birkar, C.; Cascini, P.; Hacon, C. D.; McKernan, J., Existence of minimal models for varieties of log general type, J. Am. Math. Soc., 23, 2, 405-468, 2010 · Zbl 1210.14019 · doi:10.1090/S0894-0347-09-00649-3
[13] Boutot, J.-F., Singularités rationnelles et quotients par les groupes réductifs, Invent. Math., 88, 1, 65-68, 1987 · Zbl 0619.14029 · doi:10.1007/BF01405091
[14] Braun, L., The local fundamental group of a Kawamata log terminal singularity is finite, Invent. Math., 226, 3, 845-896, 2021 · Zbl 1479.14029 · doi:10.1007/s00222-021-01062-0
[15] Braun, L., Gorensteinness and iteration of Cox rings for Fano type varieties, Math. Z., 301, 1, 1047-1061, 2022 · Zbl 1485.14072 · doi:10.1007/s00209-021-02946-w
[16] Braun, L.; Moraga, J., Iteration of Cox rings of klt singularities, J. Topol., 17, 1, 2024 · Zbl 07824475 · doi:10.1112/topo.12321
[17] Braun, L.; Filipazzi, S.; Moraga, J.; Svaldi, R., The Jordan property for local fundamental groups, Geom. Topol., 26, 1, 283-319, 2022 · Zbl 1496.14004 · doi:10.2140/gt.2022.26.283
[18] Braun, L., Greb, D., Langlois, K., Moraga, J.: Reductive quotients of klt singularities (2021), http://arxiv.org/abs/arXiv:2111.02812v1. Preprint version of this article
[19] Brion, M., Differential forms on quotients by reductive group actions, Proc. Am. Math. Soc., 126, 9, 2535-2539, 1998 · Zbl 0901.14029 · doi:10.1090/S0002-9939-98-04320-2
[20] Brion, M., The cone of effective one-cycles of certain \(G\)-varieties, A Tribute to C. S. Seshadri (Chennai, 2002), 180-198, 2003, Basel: Birkhäuser, Basel · Zbl 1069.14052 · doi:10.1007/978-93-86279-11-8_14
[21] Brion, M., Algebraic group actions on normal varieties, Trans. Mosc. Math. Soc., 78, 85-107, 2017 · Zbl 1397.14062 · doi:10.1090/mosc/263
[22] Brown, M., Singularities of Cox rings of Fano varieties, J. Math. Pures Appl. (9), 99, 6, 655-667, 2013 · Zbl 1432.14013 · doi:10.1016/j.matpur.2012.10.003
[23] Carvajal-Rojas, J.; Schwede, K.; Tucker, K., Fundamental groups of \(F\)-regular singularities via \(F\)-signature, Ann. Sci. Éc. Norm. Supér. (4), 51, 4, 993-1016, 2018 · Zbl 1408.13015 · doi:10.24033/asens.2370
[24] Cascini, P.; Spicer, C., MMP for co-rank one foliations on threefolds, Invent. Math., 225, 2, 603-690, 2021 · Zbl 1492.14025 · doi:10.1007/s00222-021-01037-1
[25] Castravet, A.-M.; Tevelev, J., \( \overline{M}_{0,n}\) is not a Mori dream space, Duke Math. J., 164, 8, 1641-1667, 2015 · Zbl 1343.14013 · doi:10.1215/00127094-3119846
[26] Cox, D. A.; Little, J. B.; Schenck, H. K., Toric Varieties, 2011, Providence: Am. Math. Soc., Providence · Zbl 1223.14001 · doi:10.1090/gsm/124
[27] de Fernex, T.; Hacon, C. D., Singularities on normal varieties, Compos. Math., 145, 2, 393-414, 2009 · Zbl 1179.14003 · doi:10.1112/S0010437X09003996
[28] Drézet, J.-M., Luna’s slice theorem and applications, Algebraic Group Actions and Quotients, 39-89, 2004, Cairo: Hindawi Publ. Corp., Cairo · Zbl 1109.14307
[29] Filipazzi, S.; Moraga, J., Strong \((\delta ,n)\)-complements for semi-stable morphisms, Doc. Math., 25, 1953-1996, 2020 · Zbl 1455.14030 · doi:10.4171/DM/790
[30] Franzen, H.; Reineke, M.; Sabatini, S., Fano quiver moduli, Can. Math. Bull., 64, 4, 984-1000, 2021 · Zbl 1485.16012 · doi:10.4153/S0008439520001009
[31] Fujino, O.; Gongyo, Y., On canonical bundle formulas and subadjunctions, Mich. Math. J., 61, 2, 255-264, 2012 · Zbl 1260.14010 · doi:10.1307/mmj/1339011526
[32] Gongyo, Y.; Okawa, S.; Sannai, A.; Takagi, S., Characterization of varieties of Fano type via singularities of Cox rings, J. Algebraic Geom., 24, 1, 159-182, 2015 · Zbl 1444.14044 · doi:10.1090/S1056-3911-2014-00641-X
[33] Greb, D., Projectivity of analytic Hilbert and Kähler quotients, Trans. Am. Math. Soc., 362, 6, 3243-3271, 2010 · Zbl 1216.14045 · doi:10.1090/S0002-9947-10-05000-2
[34] Greb, D., Rational singularities and quotients by holomorphic group actions, Ann. Sc. Norm. Super. Pisa, Cl. Sci. (5), 10, 2, 413-426, 2011 · Zbl 1241.32017 · doi:10.2422/2036-2145.2011.2.07
[35] Greb, D., Complex-analytic quotients of algebraic \(G\)-varieties, Math. Ann., 363, 1-2, 77-100, 2015 · Zbl 1398.32026 · doi:10.1007/s00208-014-1163-y
[36] Greb, D.; Kebekus, S.; Peternell, T.; Taji, B., Nonabelian Hodge theory for klt spaces and descent theorems for vector bundles, Compos. Math., 155, 2, 289-323, 2019 · Zbl 1443.14009 · doi:10.1112/s0010437x18007923
[37] Grosshans, F. D., The invariants of unipotent radicals of parabolic subgroups, Invent. Math., 73, 1, 1-9, 1983 · doi:10.1007/BF01393822
[38] Grosshans, F. D., Finitely generated rings of invariants having rational singularities, Group Actions and Invariant Theory, 53-60, 1989, Providence: Am. Math. Soc., Providence · Zbl 0716.14004
[39] Hara, N.; Watanabe, K.-I., F-regular and F-pure rings vs. log terminal and log canonical singularities, J. Algebraic Geom., 11, 2, 363-392, 2002 · Zbl 1013.13004 · doi:10.1090/S1056-3911-01-00306-X
[40] Hashimoto, M., Good filtrations and strong \(F\)-regularity of the ring of \(U_P\)-invariants, J. Algebra, 370, 198-220, 2012 · Zbl 1268.13003 · doi:10.1016/j.jalgebra.2012.07.045
[41] Hassett, B.; Hyeon, D., Log minimal model program for the moduli space of stable curves: the first flip, Ann. Math. (2), 177, 3, 911-968, 2013 · Zbl 1273.14034 · doi:10.4007/annals.2013.177.3.3
[42] Hausen, J., Geometric invariant theory based on Weil divisors, Compos. Math., 140, 6, 1518-1536, 2004 · Zbl 1072.14057 · doi:10.1112/S0010437X04000867
[43] Hausen, J.; Süß, H., The Cox ring of an algebraic variety with torus action, Adv. Math., 225, 2, 977-1012, 2010 · Zbl 1248.14008 · doi:10.1016/j.aim.2010.03.010
[44] Hausen, J.; Keicher, S.; Laface, A., On blowing up the weighted projective plane, Math. Z., 290, 3-4, 1339-1358, 2018 · Zbl 1401.14143 · doi:10.1007/s00209-018-2065-6
[45] Heinzner, P.; Huckleberry, A., Analytic Hilbert quotients, Several Complex Variables, 309-349, 1999, Cambridge: Cambridge University Press, Cambridge · Zbl 0959.32013
[46] Heinzner, P.; Loose, F., Reduction of complex Hamiltonian \(G\)-spaces, Geom. Funct. Anal., 4, 3, 288-297, 1994 · Zbl 0816.53018 · doi:10.1007/BF01896243
[47] Hochenegger, A., Martinengo, E., Tonini, F.: Cox ring of an algebraic stack (2020). http://arxiv.org/abs/arXiv:2004.01445 · Zbl 07851387
[48] Hochster, M., Book review: Cohen-Macaulay rings, Bull. Am. Math. Soc. (N.S.), 32, 2, 265-275, 1995 · doi:10.1090/S0273-0979-1995-00577-X
[49] Hollander, S., Characterizing Artin stacks, Math. Z., 269, 1-2, 467-494, 2011 · Zbl 1236.14002 · doi:10.1007/s00209-010-0746-x
[50] Howard, B.; Millson, J.; Snowden, A.; Vakil, R., The ideal of relations for the ring of invariants of \(n\) points on the line, J. Eur. Math. Soc., 14, 1, 1-60, 2012 · Zbl 1255.14039 · doi:10.4171/JEMS/295
[51] Hu, Y.; Keel, S., Mori dream spaces and GIT, Mich. Math. J., 48, 331-348, 2000 · Zbl 1077.14554 · doi:10.1307/mmj/1030132722
[52] Huneke, C., Tight Closure and Its Applications, 1996, Providence: Am. Math. Soc., Providence · Zbl 0930.13004 · doi:10.1016/0167-4889(95)00136-0
[53] Huneke, C., Hochster, M.: Tight closure in equal characteristic zero (1999). (Unpublished book draft). http://www.math.lsa.umich.edu/ hochster/tcz.ps
[54] Kapovich, M.; Kollár, J., Fundamental groups of links of isolated singularities, J. Am. Math. Soc., 27, 4, 929-952, 2014 · Zbl 1307.14005 · doi:10.1090/S0894-0347-2014-00807-9
[55] Kapranov, M. M., Chow quotients of Grassmannians. I, I. M. Gelfand Seminar, 29-110, 1993, Providence: Am. Math. Soc., Providence · Zbl 0811.14043 · doi:10.1090/advsov/016.2/02
[56] Kawamata, Y.; Okawa, S., Mori dream spaces of Calabi-Yau type and log canonicity of Cox rings, J. Reine Angew. Math., 701, 195-203, 2015 · Zbl 1314.14080 · doi:10.1515/crelle-2013-0029
[57] Kawamata, Y.; Matsuda, K.; Matsuki, K., Introduction to the minimal model problem, Algebraic Geometry, 283-360, 1987, Amsterdam: North-Holland, Amsterdam · Zbl 0672.14006 · doi:10.2969/aspm/01010283
[58] Keel, S.; McKernan, J.; Farkas, G.; Morrison, I., Contractible extremal rays on \(\overline{M}_{0,n} \), Handbook of Moduli, Vol. II, 115-130, 2013, Somerville: International Press, Somerville · Zbl 1322.14050
[59] Kempf, G. R., On the collapsing of homogeneous bundles, Invent. Math., 37, 3, 229-239, 1976 · Zbl 0338.14015 · doi:10.1007/BF01390321
[60] King, A. D., Moduli of representations of finite-dimensional algebras, Quart. J. Math. Oxf. Ser. (2), 45, 180, 515-530, 1994 · Zbl 0837.16005 · doi:10.1093/qmath/45.4.515
[61] Kirillov, A. Jr., Quiver Representations and Quiver Varieties, 2016, Providence: Am. Math. Soc., Providence · Zbl 1355.16002 · doi:10.1090/gsm/174
[62] Kodaira, K., Complex Manifolds and Deformation of Complex Structures, 1986, New York: Springer, New York · Zbl 0581.32012 · doi:10.1007/978-1-4613-8590-5
[63] Kollár, J., Singularities of pairs, Algebraic Geometry—Santa Cruz 1995, 221-287, 1997, Providence: Am. Math. Soc., Providence · Zbl 0905.14002 · doi:10.1090/pspum/062.1/1492525
[64] Kollár, J., A local version of the Kawamata-Viehweg vanishing theorem, Pure Appl. Math. Q., 7, 4, 1477-1494, 2011 · Zbl 1316.14039 · doi:10.4310/PAMQ.2011.v7.n4.a18
[65] Kollár, J., Moduli of varieties of general type, Handbook of Moduli. Vol. II, 131-157, 2013, Somerville: Int. Press, Somerville · Zbl 1322.14006
[66] Kollár, J., Singularities of the Minimal Model Program, 2013, Cambridge: Cambridge University Press, Cambridge · Zbl 1282.14028 · doi:10.1017/CBO9781139547895
[67] Kollár, J.; Mori, S., Birational Geometry of Algebraic Varieties, 1998, Cambridge: Cambridge University Press, Cambridge · Zbl 0926.14003 · doi:10.1017/CBO9780511662560
[68] Kovács, S., Rational, log canonical, Du Bois singularities. II. Kodaira vanishing and small deformations, Compos. Math., 121, 3, 297-304, 2000 · Zbl 0962.14012 · doi:10.1023/A:1001830707422
[69] Kovács, S. J., A characterization of rational singularities, Duke Math. J., 102, 2, 187-191, 2000 · Zbl 0973.14001 · doi:10.1215/S0012-7094-00-10221-9
[70] Kresch, A., On the geometry of Deligne-Mumford stacks, Algebraic Geometry—Seattle 2005. Part 1, 259-271, 2009, Providence: Am. Math. Soc., Providence · Zbl 1169.14001 · doi:10.1090/pspum/080.1/2483938
[71] Kumar, S.; Schwede, K., Richardson varieties have Kawamata log terminal singularities, Int. Math. Res. Not., 2014, 3, 842-864, 2014 · Zbl 1466.14054 · doi:10.1093/imrn/rns241
[72] Li, C.; Wang, X.; Xu, C., Quasi-projectivity of the moduli space of smooth Kähler-Einstein Fano manifolds, Ann. Sci. Éc. Norm. Supér. (4), 51, 3, 739-772, 2018 · Zbl 1421.32033 · doi:10.24033/asens.2365
[73] Li, C.; Wang, X.; Xu, C., On the proper moduli spaces of smoothable Kähler-Einstein Fano varieties, Duke Math. J., 168, 8, 1387-1459, 2019 · Zbl 1469.14087 · doi:10.1215/00127094-2018-0069
[74] Li, C.; Liu, Y.; Xu, C., A guided tour to normalized volume, Geometric Analysis, 167-219, 2020, Cham: Springer, Cham · Zbl 1440.14015 · doi:10.1007/978-3-030-34953-0_10
[75] Liendo, A.; Süss, H., Normal singularities with torus actions, Tohoku Math. J. (2), 65, 1, 105-130, 2013 · Zbl 1267.14046 · doi:10.2748/tmj/1365452628
[76] Liu, Y.; Xu, C.; Zhuang, Z., Finite generation for valuations computing stability thresholds and applications to K-stability, Ann. Math. (2), 196, 2, 507-566, 2022 · Zbl 1503.14041 · doi:10.4007/annals.2022.196.2.2
[77] Lőrincz, A. C., On the collapsing of homogeneous bundles in arbitrary characteristic, Ann. Sci. Éc. Norm. Supér. (4), 56, 5, 1313-1337, 2023 · Zbl 07827704
[78] Luna, D., Slices étales, Sur les groupes algébriques, 81-105, 1973, Memoire: MFP, Memoire · Zbl 0286.14014 · doi:10.24033/msmf.110
[79] Manivel, L., Topics on the geometry of rational homogeneous spaces, Acta Math. Sin. Engl. Ser., 36, 8, 851-872, 2020 · Zbl 1444.14039 · doi:10.1007/s10114-020-9386-1
[80] Martinez-Garcia, J.; Spotti, C., Some observations on the dimension of Fano K-moduli, Birational Geometry, Kähler-Einstein Metrics and Degenerations, 609-616, 2023, Cham: Springer, Cham · Zbl 07828765 · doi:10.1007/978-3-031-17859-7_30
[81] Moraga, J.: On a toroidalization for klt singularities (2021). http://arxiv.org/abs/arXiv:2106.15019
[82] Moraga, J., On minimal log discrepancies and Kollár components, Proc. Edinb. Math. Soc. (2), 64, 4, 982-1001, 2021 · doi:10.1017/S0013091521000729
[83] Moraga, J., Minimal log discrepancies of regularity one, Int. Math. Res. Not., 2023, 18, 15976-16014, 2023 · Zbl 1540.14035 · doi:10.1093/imrn/rnad046
[84] Mumford, D., The Red Book of Varieties and Schemes, 1999, Berlin: Springer-Verlag, Berlin · Zbl 0945.14001 · doi:10.1007/b62130
[85] Mumford, D.; Fogarty, J.; Kirwan, F., Geometric Invariant Theory, 1994, Berlin: Springer, Berlin · Zbl 0797.14004 · doi:10.1007/978-3-642-57916-5
[86] Murayama, T.: Relative vanishing theorems for \(\mathbf{Q} \)-schemes (2021). http://arxiv.org/abs/arXiv:2101.10397
[87] Nagata, M., On the 14th problem of Hilbert, Sūgaku, 12, 203-209, 1960 · Zbl 0114.36303
[88] Nemirovski, S., Levi problem and semistable quotients, Complex Var. Elliptic Equ., 58, 11, 1517-1525, 2013 · Zbl 1317.32022 · doi:10.1080/17476933.2011.592579
[89] Petersen, L.; Süss, H., Torus invariant divisors, Isr. J. Math., 182, 481-504, 2011 · Zbl 1213.14084 · doi:10.1007/s11856-011-0039-z
[90] Petracci, A.: K-moduli of Fano 3-folds can have embedded points (2021). http://arxiv.org/abs/2105.02307
[91] Petracci, A., On deformation spaces of toric singularities and on singularities of K-moduli of Fano varieties, Trans. Am. Math. Soc., 375, 8, 5617-5643, 2022 · Zbl 1502.14014 · doi:10.1090/tran/8636
[92] Popov, V. L., Picard groups of homogeneous spaces of linear algebraic groups and one-dimensional homogeneous vector fiberings, Izv. Akad. Nauk SSSR, Ser. Mat., 38, 294-322, 1974 · Zbl 0298.14023
[93] Robinson, D. J.S., A Course in the Theory of Groups, 1996, New York: Springer, New York · doi:10.1007/978-1-4419-8594-1
[94] Samuel, P., Lectures on Unique Factorization Domains, 1964, Bombay: Tata Institute of Fundamental Research, Bombay
[95] Schoutens, H., Log-terminal singularities and vanishing theorems via non-standard tight closure, J. Algebraic Geom., 14, 2, 357-390, 2005 · Zbl 1070.14005 · doi:10.1090/S1056-3911-04-00395-9
[96] Schwede, K.; Smith, K. E., Globally \(F\)-regular and log Fano varieties, Adv. Math., 224, 3, 863-894, 2010 · Zbl 1193.13004 · doi:10.1016/j.aim.2009.12.020
[97] Shokurov, V. V., Semi-stable 3-fold flips, Izv. Akad. Nauk SSSR, Ser. Mat., 57, 2, 165-222, 1993 · Zbl 0860.14016 · doi:10.1070/IM1994v042n02ABEH001541
[98] Sjamaar, R., Holomorphic slices, symplectic reduction and multiplicities of representations, Ann. Math. (2), 141, 1, 87-129, 1995 · Zbl 0827.32030 · doi:10.2307/2118628
[99] Smith, K. E., Globally F-regular varieties: applications to vanishing theorems for quotients of Fano varieties, Mich. Math. J., 48, 553-572, 2000 · Zbl 0994.14012 · doi:10.1307/mmj/1030132733
[100] Takagi, S., Yamaguchi, T.: On the behavior of adjoint ideals under pure morphisms (2023). http://arxiv.org/abs/arXiv:2312.17537
[101] Vinberg, E. B.; Popov, V. L., Invariant theory, Algebraic Geometry. IV, 123-287, 1994, Berlin: Springer, Berlin · Zbl 0789.14008 · doi:10.1007/978-3-662-03073-8_2
[102] Weyman, J., Cohomology of Vector Bundles and Syzygies, 2003, Cambridge: Cambridge University Press, Cambridge · Zbl 1075.13007 · doi:10.1017/CBO9780511546556
[103] Xu, C., Finiteness of algebraic fundamental groups, Compos. Math., 150, 3, 409-414, 2014 · Zbl 1291.14057 · doi:10.1112/S0010437X13007562
[104] Xu, C., K-stability of Fano varieties: an algebro-geometric approach, EMS Surv. Math. Sci., 8, 1, 265-354, 2021 · Zbl 1476.14030 · doi:10.4171/EMSS/51
[105] Zhuang, Z.: Direct summands of klt singularities (2022). http://arxiv.org/abs/arXiv:2208.12418
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