Dimension, multiplicity, holonomic modules, and an analogue of the inequality of Bernstein for rings of differential operators in prime characteristic
HTML articles powered by AMS MathViewer
- by V. V. Bavula
- Represent. Theory 13 (2009), 182-227
- DOI: https://doi.org/10.1090/S1088-4165-09-00352-5
- Published electronically: May 22, 2009
- PDF | Request permission
Abstract:
Let $K$ be an arbitrary field of characteristic $p>0$ and $\mathcal D (P_n)$ the ring of differential operators on a polynomial algebra $P_n$ in $n$ variables. A long anticipated analogue of the inequality of Bernstein is proved for the ring $\mathcal D (P_n)$. In fact, three different proofs are given of this inequality (two of which are essentially characteristic free): the first one is based on the concept of the filter dimension, the second, on the concept of a set of holonomic subalgebras with multiplicity, and the third works only for finitely presented modules and follows from a description of these modules (obtained in the paper). On the way, analogues of the concepts of (Gelfand-Kirillov) dimension, multiplicity, holonomic modules are found in prime characteristic (giving answers to old questions of how to find such analogs). The idea is very simple to find characteristic free generalizations (and proofs) which in characteristic zero give known results, and in prime characteristic, generalizations. An analogue of Quillen’s Lemma is proved for simple finitely presented $\mathcal D (P_n)$-modules. Moreover, for each such module $L$, $\textrm {End}_{\mathcal D (P_n)}(L)$ is a finite separable field extension of $K$ and $\dim _K(\textrm {End}_{\mathcal D (P_n)}(L))$ is equal to the multiplicity $e(L)$ of $L$. In contrast to the characteristic zero case where the Gelfand-Kirillov dimension of a nonzero finitely generated $\mathcal D (P_n)$-module $M$ can be any natural number from the interval $[n,2n]$, in the prime characteristic, the (new) dimension $\textrm {Dim}(M)$ can be any real number from the interval $[n,2n]$. It is proved that every holonomic module has finite length, but in contrast to the characteristic zero case it is not true that neither a nonzero finitely generated module of dimension $n$ is holonomic nor that a holonomic module is finitely presented. Some of the surprising results are: $(i)$ each simple finitely presented $\mathcal D (P_n)$-module $M$ is holonomic having the multiplicity which is a natural number (in characteristic zero rather the opposite is true, i.e. $\textrm {GK} (M)=2n-1$, as a rule), $(ii)$ the dimension $\textrm {Dim}(M)$ of a nonzero finitely presented $\mathcal D (P_n)$-module $M$ can be any natural number from the interval $[n,2n]$, $(iii)$ the multiplicity $e(M)$ exists for each finitely presented $\mathcal D (P_n)$-module $M$ and $e(M)\in \mathbb {Q}$, the multiplicity $e(M)$ is a natural number if $\textrm {Dim}(M)=n$, and can be an arbitrarily small rational number if $\textrm {Dim}(M)>n$.References
- Josep Alvarez-Montaner, Manuel Blickle, and Gennady Lyubeznik, Generators of $D$-modules in positive characteristic, Math. Res. Lett. 12 (2005), no. 4, 459–473. MR 2155224, DOI 10.4310/MRL.2005.v12.n4.a2
- V. V. Bavula, Identification of the Hilbert function and Poincaré series, and the dimension of modules over filtered rings, Izv. Ross. Akad. Nauk Ser. Mat. 58 (1994), no. 2, 19–39 (Russian, with Russian summary); English transl., Russian Acad. Sci. Izv. Math. 44 (1995), no. 2, 225–246. MR 1275900, DOI 10.1070/IM1995v044n02ABEH001595
- Vladimir Bavula, Filter dimension of algebras and modules, a simplicity criterion of generalized Weyl algebras, Comm. Algebra 24 (1996), no. 6, 1971–1992. MR 1386023, DOI 10.1080/00927879608825683
- Vladimir Bavula, Krull, Gelfand-Kirillov, and filter dimensions of simple affine algebras, J. Algebra 206 (1998), no. 1, 33–39. MR 1637240, DOI 10.1006/jabr.1997.7266
- Vladimir Bavula, Krull, Gelfand-Kirillov, filter, faithful and Schur dimensions, Infinite length modules (Bielefeld, 1998) Trends Math., Birkhäuser, Basel, 2000, pp. 149–166. MR 1789214
- V. V. Bavula, Filter dimension. Handbook of Algebra (Edited by M. Hazewinkel), vol. 4 (2006), 77–105.
- V. V. Bavula, Dimension, multiplicity, holonomic modules, and an analogue of the inequality of Bernstein for rings of differential operators in prime characteristic, II.
- Rikard Bøgvad, Some results on $\scr D$-modules on Borel varieties in characteristic $p>0$, J. Algebra 173 (1995), no. 3, 638–667. MR 1327873, DOI 10.1006/jabr.1995.1107
- I. N. Bernšteĭn, Modules over a ring of differential operators. An investigation of the fundamental solutions of equations with constant coefficients, Funkcional. Anal. i Priložen. 5 (1971), no. 2, 1–16 (Russian). MR 0290097
- Burkhard Haastert, Über Differentialoperatoren und $\textbf {D}$-Moduln in positiver Charakteristik, Manuscripta Math. 58 (1987), no. 4, 385–415 (German, with English summary). MR 894862, DOI 10.1007/BF01277602
- Craig L. Huneke and Rodney Y. Sharp, Bass numbers of local cohomology modules, Trans. Amer. Math. Soc. 339 (1993), no. 2, 765–779. MR 1124167, DOI 10.1090/S0002-9947-1993-1124167-6
- Günter R. Krause and Thomas H. Lenagan, Growth of algebras and Gelfand-Kirillov dimension, Revised edition, Graduate Studies in Mathematics, vol. 22, American Mathematical Society, Providence, RI, 2000. MR 1721834, DOI 10.1090/gsm/022
- Gennady Lyubeznik, $F$-modules: applications to local cohomology and $D$-modules in characteristic $p>0$, J. Reine Angew. Math. 491 (1997), 65–130. MR 1476089, DOI 10.1515/crll.1997.491.65
- Hideyuki Matsumura, Commutative ring theory, 2nd ed., Cambridge Studies in Advanced Mathematics, vol. 8, Cambridge University Press, Cambridge, 1989. Translated from the Japanese by M. Reid. MR 1011461
- J. C. McConnell and J. C. Robson, Noncommutative Noetherian rings, Pure and Applied Mathematics (New York), John Wiley & Sons, Ltd., Chichester, 1987. With the cooperation of L. W. Small; A Wiley-Interscience Publication. MR 934572
- Z. Mebkhout and L. Narváez-Macarro, Sur les coefficients de de Rham-Grothendieck des variétés algébriques, $p$-adic analysis (Trento, 1989) Lecture Notes in Math., vol. 1454, Springer, Berlin, 1990, pp. 267–308 (French). MR 1094858, DOI 10.1007/BFb0091144
- Richard S. Pierce, Associative algebras, Studies in the History of Modern Science, vol. 9, Springer-Verlag, New York-Berlin, 1982. MR 674652, DOI 10.1007/978-1-4757-0163-0
- Karen E. Smith, The $D$-module structure of $F$-split rings, Math. Res. Lett. 2 (1995), no. 4, 377–386. MR 1355702, DOI 10.4310/MRL.1995.v2.n4.a1
- Karen E. Smith and Michel Van den Bergh, Simplicity of rings of differential operators in prime characteristic, Proc. London Math. Soc. (3) 75 (1997), no. 1, 32–62. MR 1444312, DOI 10.1112/S0024611597000257
- S. P. Smith, Differential operators on commutative algebras, Ring theory (Antwerp, 1985) Lecture Notes in Math., vol. 1197, Springer, Berlin, 1986, pp. 165–177. MR 859394, DOI 10.1007/BFb0076323
Bibliographic Information
- V. V. Bavula
- Affiliation: Department of Pure Mathematics, University of Sheffield, Hicks Building, Sheffield S3 7RH, United Kingdom
- MR Author ID: 293812
- Email: v.bavula@sheffield.ac.uk
- Received by editor(s): February 27, 2008
- Published electronically: May 22, 2009
- © Copyright 2009 American Mathematical Society
- Journal: Represent. Theory 13 (2009), 182-227
- MSC (2000): Primary 13N10, 16S32, 16P90, 16D30, 16W70
- DOI: https://doi.org/10.1090/S1088-4165-09-00352-5
- MathSciNet review: 2506264
Dedicated: Dedicated to Joseph Bernstein on the occasion of his 60th birthday