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Lenhard Lee Ng

Professor of Mathematics
Mathematics
216 Physics Building, Box 90320, Durham, NC 27708-0320
120 Science Drive, Durham, NC 27708

Selected Publications


Braid loops with infinite monodromy on the Legendrian contact DGA

Journal Article Journal of Topology · December 1, 2022 We present the first examples of elements in the fundamental group of the space of Legendrian links in (Formula presented.) whose action on the Legendrian contact DGA is of infinite order. This allows us to construct the first families of Legendrian links ... Full text Open Access Cite

Augmentations are sheaves

Journal Article Geometry and Topology · January 1, 2020 Full text Open Access Cite

Higher genus knot contact homology and recursion for colored HOMFLY-PT polynomials

Journal Article ADVANCES IN THEORETICAL AND MATHEMATICAL PHYSICS · 2020 Open Access Cite

Higher genus knot contact homology and recursion for colored HOMFLY-PT polynomials

Journal Article Advances in Theoretical and Mathematical Physics · January 1, 2020 We sketch a construction of Legendrian Symplectic Field Theory (SFT) for conormal tori of knots and links. Using large N duality and Witten’s connection between open Gromov–Witten invariants and Chern–Simons gauge theory, we relate the SFT of a link conorm ... Full text Open Access Cite

Legendrian contact homology in $\mathbb{R}^3$

Journal Article Surveys in Differential Geometry · 2020 Full text Cite

Representations, sheaves and Legendrian (2,m) torus links

Journal Article JOURNAL OF THE LONDON MATHEMATICAL SOCIETY-SECOND SERIES · August 1, 2019 Full text Open Access Link to item Cite

A complete knot invariant from contact homology

Journal Article Inventiones Mathematicae · March 1, 2018 We construct an enhanced version of knot contact homology, and show that we can deduce from it the group ring of the knot group together with the peripheral subgroup. In particular, it completely determines a knot up to smooth isotopy. The enhancement cons ... Full text Open Access Cite

Problems and solutions

Journal Article American Mathematical Monthly · January 1, 2018 Full text Cite

Problems and solutions

Journal Article American Mathematical Monthly · December 1, 2017 Full text Cite

Problems and solutions

Journal Article American Mathematical Monthly · October 1, 2017 Full text Cite

Problems and solutions

Journal Article American Mathematical Monthly · May 1, 2017 Full text Cite

Knot contact homology, string topology, and the cord algebra

Journal Article Journal de l'Ecole Polytechnique - Mathematiques · January 1, 2017 The conormal Lagrangian LK of a knot K in R3 is the submanifold of the cotangent bundle T∗R3 consisting of covectors along K that annihilate tangent vectors to K. By intersecting with the unit cotangent bundle S∗R3, one obtains the unit conormal ΛK, and th ... Full text Open Access Cite

Problems and Solutions

Journal Article The American Mathematical Monthly · 2017 Full text Cite

Obstructions to Lagrangian concordance

Journal Article Algebraic & Geometric Topology · April 26, 2016 Full text Open Access Cite

On transverse invariants from Khovanov homology

Journal Article Quantum Topology · September 9, 2015 In [31], O. Plamenevskaya associated to each transverse knot K an element of the Khovanov homology of K. In this paper, we give two re_nements of Plamenevskaya’s invariant, one valued in Bar-Natan’s deformation (from [2]) of the Khovanov complex and anothe ... Full text Open Access Cite

Legendrian contact homology in the boundary of a subcritical weinstein 4-Manifold

Journal Article Journal of Differential Geometry · September 1, 2015 We give a combinatorial description of the Legendrian contact homology algebra associated to a Legendrian link in S1 × S2 or any connected sum #k(S1 ×S2), viewed as the contact boundary of the Weinstein manifold obtained by attaching 1-handles to the 4-bal ... Full text Open Access Cite

Topological strings, D-model, and knot contact homology

Journal Article Advances in Theoretical and Mathematical Physics · January 1, 2014 We study the connection between topological strings and contact homology recently proposed in the context of knot invariants. In particular, we establish the proposed relation between the Gromov- Witten disk amplitudes of a Lagrangian associated to a knot ... Full text Open Access Cite

A topological introduction to knot contact homology

Journal Article Bolyai Society Mathematical Studies · January 1, 2014 Full text Open Access Cite

Satellites of Legendrian knots and representations of the Chekanov-Eliashberg Algebra

Journal Article Algebraic and Geometric Topology · August 1, 2013 We develop a close relation between satellites of Legendrian knots in ℝ3 and the Chekanov-Eliashberg differential graded algebra of the knot. In particular, we generalize the well-known correspondence between rulings of a Legendrian knot in ℝ3 and augmenta ... Full text Open Access Cite

Knot contact homology

Journal Article Geometry and Topology · May 13, 2013 The conormal lift of a link K in ℝ3 is a Legendrian submanifold ∧K in the unit cotangent bundle U*ℝ3 of ℝ3 with contact structure equal to the kernel of the Liouville form. Knot contact homology, a topological link invariant of K, is defined as the Legendr ... Full text Open Access Cite

Filtrations on the knot contact homology of transverse knots

Journal Article Mathematische Annalen · April 1, 2013 We construct a new invariant of transverse links in the standard contact structure on ℝ3 This invariant is a doubly filtered version of the knot contact homology differential graded algebra (DGA) of the link, see (Ekholm et al., Knot contact homology, Arxi ... Full text Cite

On arc index and maximal Thurston-Bennequin number

Journal Article Journal of Knot Theory and Its Ramifications · 2012 Open Access Cite

Combinatorial knot contact homology and transverse knots

Journal Article Advances in Mathematics · August 20, 2011 We give a combinatorial treatment of transverse homology, a new invariant of transverse knots that is an extension of knot contact homology. The theory comes in several flavors, including one that is an invariant of topological knots and produces a three-v ... Full text Cite

An atlas of Legendrian knots

Journal Article Experimental Mathematics · 2011 Cite

Legendrian and transverse twist knots

Journal Article Journal of the European Mathematical Society · 2011 Cite

Rational symplectic field theory for Legendrian knots

Journal Article Inventiones Mathematicae · December 1, 2010 We construct a combinatorial invariant of Legendrian knots in standard contact three-space. This invariant, which encodes rational relative Symplectic Field Theory and extends contact homology, counts holomorphic disks with an arbitrary number of positive ... Full text Cite

A family of transversely nonsimple knots

Journal Article Algebraic and Geometric Topology · July 28, 2010 We apply knot Floer homology to exhibit an infinite family of transversely nonsimple prime knots starting with 10132. We also discuss the combinatorial relationship between grid diagrams, braids and Legendrian and transverse knots in standard contact R3. ... Full text Cite

Grid Diagrams, Braids, and Contact Geometry

Journal Article Proceedings of Gokova Geometry-Topology Conference 2008, 120--136 · December 19, 2008 We use grid diagrams to present a unified picture of braids, Legendrian knots, and transverse knots. ... Link to item Cite

A skein approach to bennequin-type inequalities

Journal Article International Mathematics Research Notices · December 1, 2008 We give a simple unified proof for several disparate bounds on Thurston-Bennequin number for Legendrian knots and self-linking number for transverse knots in 3, and provide a template for possible future bounds. As an application, we give sufficient condit ... Full text Cite

Framed knot contact homology

Journal Article Duke Mathematical Journal · February 1, 2008 We extend knot contact homology to a theory over the ring ℤ[λ±1, μ±1] with the invariant given topologically and combinatorially. The improved invariant, which is defined for framed knots in S3 and can be generalized to knots in arbitrary manifolds, distin ... Full text Cite

Transverse knots distinguished by knot Floer homology

Journal Article Journal of Symplectic Geometry · 2008 Cite

Conormal bundles, contact homology and knot invariants

Journal Article Geometry and Topology Monographs · April 22, 2006 Full text Cite

The correspondence between augmentations and rulings for legendrian knots

Journal Article Pacific Journal of Mathematics · March 1, 2006 We strengthen the link between holomorphic and generating-function invariants of Legendrian knots by establishing a formula relating the number of augmentations of a knot's contact homology to the complete ruling invariant of Chekanov and Pushkar. ... Full text Cite

Insights from SMPY's greatest former child prodigies: Drs. Terence ("Terry") Tao and Lenhard ("Lenny") Ng reflect on their talent development

Journal Article Gifted Child Quarterly · January 1, 2006 If the academic needs of the most profoundly gifted students can be met through the use of existing educational practices, specialists in gifted education can assume that the educational needs of less able, but still academically talented, students can als ... Full text Cite

Knot and braid invariants from contact homology II

Journal Article Geometry and Topology · August 26, 2005 We present a topological interpretation of knot and braid contact homology in degree zero, in terms of cords and skein relations. This interpretation allows us to extend the knot invariant to embedded graphs and higher-dimensional knots. We calculate the k ... Cite

Plane curves and contact geometry

Journal Article Proceedings of G\~A{\P}kova Geometry-Topology Conference 2005 (2006), 165-174 · March 8, 2005 We apply contact homology to obtain new results in the problem of distinguishing immersed plane curves without dangerous self-tangencies. ... Link to item Cite

Knot and braid invariants from contact homology I

Journal Article Geometry and Topology · January 26, 2005 We introduce topological invariants of knots and braid conjugacy classes, in the form of differential graded algebras, and present an explicit combinatorial formulation for these invariants. The algebras conjecturally give the relative contact homology of ... Full text Cite

Legendrian solid-torus links

Journal Article Journal of Symplectic Geometry · 2005 Cite

A Legendrian Thurston–Bennequin bound from Khovanov homology

Journal Article Algebraic & Geometric Topology · 2005 Cite

Computable Legendrian invariants

Journal Article Topology · January 1, 2003 We establish tools to facilitate the computation and application of the Chekanov-Eliashberg differential graded algebra (DGA), a Legendrian-isotopy invariant of Legendrian knots and links in standard contact three space. More specifically, we reformulate t ... Full text Cite

Invariants of Legendrian links and coherent orientations

Journal Article J. Symplectic Geom. · 2002 Cite

The Legendrian satellite construction

Journal Article Appears as part of"Legendrian solid-torus links", J. Symplectic Geom. 2 (2005), No. 3, 411-443 · December 11, 2001 We examine the Legendrian analogue of the topological satellite construction for knots, and deduce some results for specific Legendrian knots and links in standard contact three-space and the solid torus. In particular, we show that the Chekanov-Eliashberg ... Link to item Cite

Maximal Thurston–Bennequin number of two-bridge links

Journal Article Algebr. Geom. Topol. · 2001 Cite

Legendrian mirrors and Legendrian isotopy

Journal Article · August 28, 2000 We resolve a question of Fuchs and Tabachnikov by showing that there is a Legendrian knot in standard contact three-space with zero Maslov number which is not Legendrian isotopic to its mirror. The proof uses the differential graded algebras of Chekanov. ... Link to item Cite

Hamiltonian Decomposition of Lexicographic Products of Digraphs

Journal Article Journal of Combinatorial Theory. Series B · July 1, 1998 We partially resolve a conjecture of Alspach, Bermond, and Sotteau by showing that the lexicographic product of two hamiltonian decomposable digraphs, the first of odd order, is itself hamiltonian decomposable in general. © 1998 Academic Press. ... Full text Cite

The rook on the half-chessboard, or how not to diagonalize a matrix

Journal Article American Mathematical Monthly · January 1, 1998 Full text Cite

Hamiltonian decomposition of complete regular multipartite digraphs

Journal Article Discrete Mathematics · December 1, 1997 We prove that the complete regular multipartite digraph K*r:s is decomposable into directed hamiltonian cycles if and only if (r,s) ≠ (4,1) or (6,1), thereby answering a question of Alspach, Bermond, and Sotteau. © 1997 Elsevier Science B.V. ... Full text Cite

k-Ordered Hamiltonian Graphs

Journal Article Journal of Graph Theory · January 1, 1997 A hamiltonian graph G of order n is k-ordered, 2 ≤ k ≤ n, if for every sequence v1, v2, . . . , vk of k distinct vertices of G, there exists a hamiltonian cycle that encounters v1, v2, . . . , vk in this order. Theorems by Dirac and Ore, presenting suffici ... Full text Cite