Zlil Sela () is an Israeli mathematician working in the area of geometric group theory. He is a Professor of Mathematics at the Hebrew University of Jerusalem. Sela is known for the solution of the isomorphism problem for torsion-free word-hyperbolic groups and for the solution of the Tarski conjecture about equivalence of first-order theories of finitely generated non-abelian free groups.
Biographical data Sela received his Ph.D. in 1991 from the Hebrew University of Jerusalem, where his doctoral advisor was Eliyahu Rips. Prior to his current appointment at the Hebrew University, he held an Associate Professor position at Columbia University in New York.Faculty Members Win Fellowships Columbia University Record, May 15, 1996, Vol. 21, No. 27. While at Columbia, Sela won the Sloan Fellowship from the Sloan Foundation.Sloan Fellowships Awarded Notices of the American Mathematical Society, vol. 43 (1996), no. 7, pp. 781–782
Sela gave an Invited Address at the 2002 International Congress of Mathematicians in Beijing.Z. Sela. Diophantine geometry over groups and the elementary theory of free and hyperbolic groups. Proceedings of the International Congress of Mathematicians, Vol. II (Beijing, 2002), pp. 87 92, Higher Ed. Press, Beijing, 2002. Invited Speakers for ICM2002. Notices of the American Mathematical Society, vol. 48, no. 11, December 2001; pp. 1343 1345 He gave a plenary talk at the 2002 annual meeting of the Association for Symbolic Logic,The 2002 annual meeting of the Association for Symbolic Logic. Bulletin of Symbolic Logic, vol. 9 (2003), pp. 51–70 and he delivered an AMS Invited Address at the October 2003 meeting of the American Mathematical SocietyAMS Meeting at Binghamton, New York. Notices of the American Mathematical Society, vol. 50 (2003), no. 9, p. 1174 and the 2005 Tarski Lectures at the University of California at Berkeley.2005 Tarski Lectures. Department of Mathematics, University of California at Berkeley. Accessed September 14, 2008. He was also awarded the 2003 Erdős Prize from the Israel Mathematical Union.Erdős Prize. Israel Mathematical Union. Accessed September 14, 2008 Sela also received the 2008 Carol Karp Prize from the Association for Symbolic Logic for his work on the Tarski conjecture and on discovering and developing new connections between model theory and geometric group theory.Karp Prize Recipients. Association for Symbolic Logic. Accessed September 13, 2008ASL Karp and Sacks Prizes Awarded , Notices of the American Mathematical Society, vol. 56 (2009), no. 5, p. 638
Mathematical contributions Sela's early important work was his solutionZ. Sela. "The isomorphism problem for hyperbolic groups. I." Annals of Mathematics (2), vol. 141 (1995), no. 2, pp. 217–283. in mid-1990s of the isomorphism problem for torsion-free word-hyperbolic groups. The machinery of group actions on real trees, developed by Eliyahu Rips, played a key role in Sela's approach. The solution of the isomorphism problem also relied on the notion of canonical representatives for elements of hyperbolic groups, introduced by Rips and Sela in a joint 1995 paper.Z. Sela, and E. Rips. Canonical representatives and equations in hyperbolic groups, Inventiones Mathematicae vol. 120 (1995), no. 3, pp. 489–512 The machinery of the canonical representatives allowed Rips and Sela to prove algorithmic solvability of finite systems of equations in torsion-free hyperbolic groups, by reducing the problem to solving equations in free groups, where the Makanin–Razborov algorithm can be applied. The technique of canonical representatives was later generalized by DahmaniFrançois Dahmani. "Accidental parabolics and relatively hyperbolic groups." Israel Journal of Mathematics, vol. 153 (2006), pp. 93–127 to the case of relatively hyperbolic groups and played a key role in the solution of the isomorphism problem for toral relatively hyperbolic groups.François Dahmani, and Daniel Groves, "The isomorphism problem for toral relatively hyperbolic groups". Publications Mathématiques de l'IHÉS, vol. 107 (2008), pp. 211–290
In his work on the isomorphism problem Sela also introduced and developed the notion of a JSJ-decomposition for word-hyperbolic groups,Z. Sela. Geometric and Functional Analysis, vol. 7 (1997), no. 3, pp. 561–593 motivated by the notion of a JSJ decomposition for 3-manifolds. A JSJ-decomposition is a representation of a word-hyperbolic group as the fundamental group of a graph of groups which encodes in a canonical way all possible splittings over infinite cyclic subgroups. The idea of JSJ-decomposition was later extended by Rips and Sela to torsion-free finitely presented groupsE. Rips, and Z. Sela. "Cyclic splittings of finitely presented groups and the canonical JSJ decomposition." Annals of Mathematics (2), vol. 146 (1997), no. 1, pp. 53–109 and this work gave rise a systematic development of the JSJ-decomposition theory with many further extensions and generalizations by other mathematicians.M. J. Dunwoody, and M. E. Sageev. "JSJ-splittings for finitely presented groups over slender groups." Inventiones Mathematicae, vol. 135 (1999), no. 1, pp. 25 44P. Scott and G. A. Swarup. "Regular neighbourhoods and canonical decompositions for groups." Electronic Research Announcements of the American Mathematical Society, vol. 8 (2002), pp. 20–28B. H. Bowditch. "Cut points and canonical splittings of hyperbolic groups." Acta Mathematica, vol. 180 (1998), no. 2, pp. 145–186K. Fujiwara, and P. Papasoglu, Geometric and Functional Analysis, vol. 16 (2006), no. 1, pp. 70–125 Sela applied a combination of his JSJ-decomposition and real tree techniques to prove that torsion-free word-hyperbolic groups are Hopfian. This result and Sela's approach were later generalized by others to finitely generated subgroups of hyperbolic groupsInna Bumagina, "The Hopf property for subgroups of hyperbolic groups." Geometriae Dedicata, vol. 106 (2004), pp. 211–230 and to the setting of relatively hyperbolic groups.
Sela's most important work came in early 2000s when he produced a solution to a famous Tarski conjecture. Namely, in a long series of papers,Z. Sela. "Diophantine geometry over groups. I. Makanin-Razborov diagrams." Publications Mathématiques. Institut de Hautes Études Scientifiques, vol. 93 (2001), pp. 31–105Z. Sela. Diophantine geometry over groups. II. Completions, closures and formal solutions. Israel Journal of Mathematics, vol. 134 (2003), pp. 173–254Z. Sela. Israel Journal of Mathematics, vol. 147 (2005), pp. 1–73Z. Sela. Israel Journal of Mathematics, vol. 143 (2004), pp. 1–130Z. Sela. Israel Journal of Mathematics, vol. 150 (2005), pp. 1–197Z. Sela. Geometric and Functional Analysis, vol. 16 (2006), no. 3, pp. 537–706Z. Sela. Geometric and Functional Analysis, vol. 16 (2006), no. 3, pp. 707–730 he proved that any two non-abelian finitely generated free groups have the same first-order theory. His work relied on applying his earlier JSJ-decomposition and real tree techniques as well as developing new ideas and machinery of "algebraic geometry" over free groups.
Sela pushed this work further to study first-order theory of arbitrary torsion-free word-hyperbolic groups and to characterize all groups that are elementarily equivalent to (that is, have the same first-order theory as) a given torsion-free word-hyperbolic group. In particular, his work implies that if a finitely generated group G is elementarily equivalent to a word-hyperbolic group then G is word-hyperbolic as well.
Sela also proved that the first-order theory of a finitely generated free group is stable in the model-theoretic sense, providing a brand-new and qualitatively different source of examples for the stability theory.
An alternative solution for the Tarski conjecture has been presented by Olga Kharlampovich and Alexei Myasnikov.O. Kharlampovich, and A. Myasnikov. "Tarski's problem about the elementary theory of free groups has a positive solution." Electronic Research Announcements of the American Mathematical Society, vol. 4 (1998), pp. 101–108O. Kharlampovich, and A. Myasnikov. Implicit function theorem over free groups. Journal of Algebra, vol. 290 (2005), no. 1, pp. 1–203O. Kharlampovich, and A. Myasnikov. "Algebraic geometry over free groups: lifting solutions into generic points." Groups, languages, algorithms, pp. 213–318, Contemporary Mathematics, vol. 378, American Mathematical Society, Providence, RI, 2005O. Kharlampovich, and A. Myasnikov. "Elementary theory of free non-abelian groups." Journal of Algebra, vol. 302 (2006), no. 2, pp. 451–552
The work of Sela on first-order theory of free and word-hyperbolic groups substantially influenced the development of geometric group theory, in particular by stimulating the development and the study of the notion of limit groups and of relatively hyperbolic groups.Frédéric Paulin. Sur la théorie élémentaire des groupes libres (d'après Sela). Astérisque No. 294 (2004), pp. 63–402
Sela's classification theorem Theorem. Two non-abelian torsion-free hyperbolic groups are elementarily equivalent if and only if their cores are isomorphic. (See p. 8.)
Published work
(Sela's theorem on acylindrical accessibility for groups)
See also Geometric group theory Stable theory Free group Word-hyperbolic group Group isomorphism problem Real trees JSJ decomposition
References
External links Zlil Sela's webpage at the Hebrew University Zlil Sela at the Mathematics Genealogy Project