Joshua P. Swanson

About me

Brief biography:

I'm an NTT RTPC Assistant Professor of Mathematics at the University of Southern California (USC) in Los Angeles working with Greta Panova. I am on the tenure-track market this Fall 2024. From Fall 2018 to Spring 2021, I was a Stefan E. Warshawski Visiting Assistant Professor at the University of California, San Diego (UCSD) under the mentorship of Brendon Rhoades. I graduated from the University of Washington in June 2018 with a PhD in Mathematics. My doctoral advisor was Sara Billey. My PhD thesis was on "Major Index Statistics: Cyclic Sieving, Branching Rules, and Asymptotics".

I am currently co-organizing:

the USC Combinatorics Seminar (weekly)
the CanaDAM 2025 session on Web Graphs (May 20-23, 2025)
the ICERM Topical Workshop on Webs in Algebra, Geometry, Topology and Combinatorics (December 8-12, 2025).

Research overview: I study algebraic combinatorics. My primary research interest involves understanding web bases for representations of quantum groups. Related topics include plabic graphs, invariant theory, crystals, the geometry of affine buildings, and certain statistical mechanics models. I also work on super coinvariant algebras, $q$-analogues, and aspects of tableau combinatorics. I have further work in analytic combinatorics related to limit laws for combinatorial statistics.

Research interests: Algebraic combinatorics, webs for quantum groups, plabic graphs, invariant theory, symmetric functions, Coxeter groups, complex reflection groups, higher coinvariant algebras, tableau combinatorics, Stirling numbers, dynamical algebraic combinatorics, $q$-analogues, limit laws, cumulants, free Lie algebras

Publications and preprints

Super major index and Thrall's problem (2024).
With Sam Armon.
Submitted. 23 pages. Available at arXiv:2411.04302.
Stirling numbers for complex reflection groups (2024).
With Bruce Sagan.
Submitted. 26 pages. Available at arXiv:2408.13874.
Web bases in degree two from hourglass plabic graphs (2024).
With Christian Gaetz, Oliver Pechenik, Stephan Pfannerer, and Jessica Striker.
Submitted. 24 pages. Available at arXiv:2402.13978.
Rotation-invariant web bases from hourglass plabic graphs (2023).
With Christian Gaetz, Oliver Pechenik, Stephan Pfannerer, and Jessica Striker.
Submitted. 65 pages. Available at arXiv:2306.12501.
Promotion permutations for tableaux (2024).
With Christian Gaetz, Oliver Pechenik, Stephan Pfannerer, and Jessica Striker.
Published in Comb. Theory 4 (2024), no. 2. 55 pages. Available at arXiv:2306.12506.
An $\mathrm{SL}_4$-web basis from hourglass plabic graphs (2023).
With Christian Gaetz, Oliver Pechenik, Stephan Pfannerer, and Jessica Striker.
Extended abstract published in Sém. Lothar. Combin, 89B (2023), Art. 9, 12 pp. 12 pages. pdf
Cyclotomic generating functions (2024).
With Sara C. Billey.
Published in Electron. J. Combin. 31 (2024), no 4. Paper 4.4. 44 pages. Available at arXiv:2305.07620.
$q$-Stirling numbers in type $B$ (2024).
With Bruce Sagan.
Published in European J. Combin. 118 (2024), Paper No. 103899. 35 pages. Available at arXiv:2205.14078.
Curious cyclic sieving on increasing tableaux (2022).
With Christian Gaetz, Oliver Pechenik, and Jessica Striker.
Published in Enumer. Combin. Appl. ECA 2:3 (2022) Article S2R17. 9 pages. Available at arXiv:2112.09228. pdf
Tanisaki witness relations for harmonic differential forms (2024).
Published in Algebr. Comb. 7 (2024), no. 1. 26 pages. Available at arXiv:2109.05080.
Harmonic differential forms for pseudo-reflection groups II. Bi-degree bounds (2023).
With Nolan R. Wallach.
Published in Comb. Theory 3 (2023), no. 3, Paper No. 17. 43 pages. Available at arXiv:2109.03407.
The metric space of limit laws of $q$-hook formulas (2022).
With Sara C. Billey.
Published in Combinatorial Theory 2 (2) (2022),. 58 pages. Available at arXiv:2010.12701. pdf
On the distribution of the major index on standard Young tableaux (2020).
With Sara C. Billey and Matjaž Konvalinka.
Talk at FPSAC 2020. Extended abstract published in Sém. Lothar. Combin, 84B (2020), Art. 44, 12 pp. Available at arXiv:2005.10341. pdf
Harmonic differential forms for pseudo-reflection groups I. Semi-invariants (2021).
With Nolan R. Wallach.
Published in J. Combin. Theory Ser. A 182 (2021), Paper No. 105474. 30 pages. Available at arXiv:2001.06076. pdf
Thrall’s problem: cyclic sieving, necklaces, and branching rules (2019).
With Connor Ahlbach.
Talk at FPSAC 2019. Extended abstract published in Sém. Lothar. Combin, 82B (2019), Art. 37, 12 pp. pdf
Existence and hardness of conveyor belts (2020).
With Molly Baird, Sara C. Billey, Erik D. Demaine, Martin L. Demaine, David Eppstein, Sándor Fekete, Graham Gordon, Sean Griffin, Joseph S. B. Mitchell.
Published in Electron. J. Combin. 27 (2020), no 4. Paper 4.25. 21 pages. Available at arXiv:1908.07668. pdf
Alternating super-polynomials and super-coinvariants of finite reflection groups (2019).
Superceded by joint work with Nolan Wallach, Harmonic differential forms for pseudo-reflection groups I. Semi-invariants. 18 pages. Available at arXiv:1908.00196.
Asymptotic normality of the major index on standard tableaux (2020).
With Sara C. Billey and Matjaž Konvalinka.
Published in Adv. in Appl. Math. 113 (2020), 101972. 28 pages. Available at arXiv:1905.00975.
On a Theorem of Baxter and Zeilberger via a Result of Roselle (2022).
Published in Ann. Comb. (2022). 9 pages. Available at arXiv:1902.06724. pdf
Tableau posets and the fake degrees of coinvariant algebras (2020).
With Sara C. Billey and Matjaž Konvalinka.
Published in Advances in Mathematics 371 (2020). 45 pages. Available at arXiv:1809.07386.
Cyclic sieving, necklaces, and branching rules related to Thrall's problem (2018).
With Connor Ahlbach.
Published in Electron. J. Combin. 25 (2018), no. 4, Paper 4.42. 38 pages. Available at arXiv:1808.06043. pdf
Major Index Statistics: Cyclic Sieving, Branching Rules, and Asymptotics (2018).
Ph.D. Thesis at the University of Washington under Sara Billey. ISBN 978-0438-52242-8. 206 pages. pdf
Refined cyclic sieving on words for the major index statistic (2018).
With Connor Ahlbach.
Published in European J. Combin. 73 (2018), 37-60. 24 pages. Available at arXiv:1706.08631.
On the Existence of Tableaux with Given Modular Major Index (2018).
Published in Algebraic Combinatorics Volume 1 (2018), no. 1 pp. 3-21. 20 pages. Available at arXiv:1701.04963. pdf
Refined cyclic sieving (2017).
With Connor Ahlbach.
Poster presentation at FPSAC 2017. Extended abstract published in Sém. Lothar. Combin, 78B (2017), Art. 48, 12 pp. pdf
Standard tableaux and modular major index (2017).
Poster presentation at FPSAC 2017. Extended abstract published in Sém. Lothar. Combin, 78B (2017), Art. 50, 9 pp. pdf

Invited talks

(11/10/2025) TBD. ICERM Workshop on Computation in Representation Theory. web
(2/21/2025) TBD. U Michigan Combinatorics Seminar. web
(11/07/2024) Webs, pockets, and buildings. UCLA Combinatorics Forum. web pdf
(10/23/2024) Writing a Math CV. USC MGSA CV Workshop. web pdf
(10/21/2024) Cyclotomic generating functions. USC Combinatorics Seminar. web
(10/10/2024) Cyclotomic generating functions. University of Waterloo Algebra and Enumerative Combinatorics Seminar. web
(4/18/2024) Unifying lattices through hourglass plabic graphs. IPAM GSMI Workshop II: Integrability and Algebraic Combinatorics. web pdf
(4/14/2024) Promotion permutations. SoCalDM 2024. web pdf
(3/23/2024) Tanisaki witness relations. CombinaTexas 2024. web pdf
(2/28/2024) $\mathrm{SL}_4$ web bases from hourglass plabic graphs. UC Berkeley Combinatorics Seminar. web pdf
(2/25/2024) Type B q-Stirling numbers. Bruce Sagan 70th Birthday Conference, University of Florida. web pdf
(10/10/2023) Type B q-Stirling numbers. North Dakota State University, Combinatorics Seminar. pdf
(8/28/2023) $\mathrm{SL}_4$ web bases from hourglass plabic graphs. University of Southern California, Combinatorics Seminar. web pdf
(5/1/2023) $\mathrm{SL}_4$ web bases from hourglass plabic graphs. University of California, Davis Algebra and Discrete Math Seminar. web pdf
(4/19/2023) $\mathrm{SL}_4$ web bases from hourglass plabic graphs. University of Washington Combinatorics Seminar. web pdf
(3/25/2023) USC Math Student Life. SCMC 2023 High School Math Competition.
(3/19/2023) Super coinvariants beyond type $A$. AMS Special Session on Macdonald Theory at Georgia Institute of Technology. web pdf video
(11/7/2022) Higher coinvariant algebras, q-Stirling numbers, and Coxeter-like complexes. Cornell Discrete Geometry and Combinatorics Seminar. web pdf
(11/2/2022) Higher coinvariant algebras, q-Stirling numbers, and Coxeter-like complexes. USC Combinatorics Seminar. web pdf
(10/26/2022) Higher coinvariant algebras, q-Stirling numbers, and Coxeter-like complexes. MIT-Harvard-MSR Combinatorics Seminar. web pdf
(10/13/2022) "Spinny pictures": connecting Catalan combinatorics, alternating sign matrices, and plane partitions. NDSU Mathematics Department Colloquium. web
(6/7/2022) q-Stirling numbers in type B. AlCoVE poster presentation. web pdf
(3/10/2022) Type B q-Stirling numbers. University of Waterloo Algebraic and Enumerative Combinatorics Seminar. web pdf
(11/16/2021) DUSTPAN distributions as limit laws for Mahonian statistics on forests. UCSD Combinatorics Seminar. web pdf
(10/7/2021) Combinatorics of harmonic polynomial differential forms. UCLA Combinatorics Seminar. web pdf
(9/27/2021) Differential Coinvariant Algebras. USC Algebra Seminar. web pdf
(3/23/2021) Cyclotomic generating functions. ICERM workshop on Geometry and Combinatorics of Root Systems. web pdf video
(2/27/2021) An Introduction to Pure Mathematics Research. UCSD Graduate Road Map 2021. pdf
(2/20/2021) DUSTPAN distributions as limit laws for Mahonian statistics on forests. CombinaTexas 2021. pdf
(1/27/2021) DUSTPAN distributions as limit laws for Mahonian statistics on forests. Michigan State University Combinatorics and Graph Theory Seminar. pdf
(12/15/2020) DUSTPAN distributions as limit laws for Mahonian statistics on forests. University of Ljubljana Discrete Math Seminar. pdf
(10/25/2020)* (Canceled due to COVID-19). BIRS Workshop on Dynamical Algebraic Combinatorics, Banff, Alberta, Canada. web
(7/13/2020) On the distribution of the major index on standard Young tableaux. FPSAC 2020 Online (Contributed). web pdf video
(6/1/2020)* Harmonic differential forms for pseudo-reflection groups. SIAM Conference on Discrete Mathematics (DM20) in Portland, Oregon. web
(5/2/2020)* Harmonic differential forms for pseudo-reflection groups. AMS Spring Western Sectional Meeting at Cal State Fresno: Special Session on Combinatorics Arising from Representations. web
(11/3/2019) Alternating super-polynomials and super-coinvariants of finite reflection groups. SIAM TX/LA Section at SMU, Dallas. web pdf
(7/2/2019) Thrall's problem: cyclic sieving, necklaces, and branching rules. FPSAC 2019, University of Ljubljana, Slovenia (Contributed). web pdf
(5/4/2019) Asymptotics of Mahonian statistics. Southern California Discrete Math Symposium (Contributed). web pdf
(3/24/2019) Cyclotomic generating function asymptotics. CombinaTexas 2019 at Texas A-M, College Station (Contributed). web pdf
(2/13/2019) Cyclotomic generating function asymptotics. USC Combinatorics Seminar. web pdf
(10/27/2018) Tableaux posets and the fake degrees of coinvariant algebras. AMS Fall Western Sectional Meeting at SFSU: Special Session on Combinatorial and Categorial Aspects of Representation Theory. web pdf
(5/23/2018) Major Index Statistics: Cyclic Sieving, Branching Rules, and Asymptotics (PhD thesis defense). University of Washington Combinatorics Seminar. web pdf
(3/26/2018) Inv and Maj Asymptotics. University of Washington Probability Seminar. web pdf
(1/19/2018) Major Index Asymptotics. University of Michigan Combinatorics Seminar. web pdf
(1/13/2018) Refined Cyclic Sieving on Words and Tableaux. JMM in San Diego: AMS Special Session on Dynamical Algebraic Combinatorics. web pdf
(11/4/2017) Major Index Asymptotics. AMS Fall Western Sectional Meeting at UC Riverside: Special Session on Combinatorial Representation Theory. web pdf
(2/27/2017) On the existence of tableaux with given modular major index. UCSD Combinatorics Seminar. pdf
(11/4/2016) Refined cyclic sieving on words. University of Minnesota Combinatorics Seminar. web pdf
(3/11/2015) Schubert multiplication rules and Bruhat chains (Candidacy exam). University of Washington Combinatorics Seminar. web pdf

*Postponed or canceled due to COVID-19

Informal talks

(10/16/2020) Asymptotics of Mahonian statistics. UCSD Student Colloquium. pdf
(11/12/2019) A gentle introduction to coinvariant algebras. UCSD Postdoc Seminar. pdf
(6/1/2018) Lehrer--Solomon cohomology decomposition. University of Washington Hyperplane Arrangements graduate course. pdf
(1/17/2018) Introduction to $\lambda$-rings and Plethysms. University of Washington CAT seminar. pdf
(10/12/2017) A zoo of coinvariant algebras. University of Washington 123 Seminar. pdf
(10/10/2017) Introduction to Hopf monoids. University of Washington CAT seminar. pdf
(5/30/2017) The Hilbert Scheme of Points in the Plane is Connected. University of Washington Applied Algebraic Geometry course. pdf
(5/1/2017) Irreducible decompositions. University of Washington Applied Algebraic Geometry course. pdf
(4/13/2017) Asymptotic normality. University of Washington CAT seminar. pdf
(4/5/2017) Monomial orderings. University of Washington Applied Algebraic Geometry course. pdf
(2/22/2017) Intro to complex reflection groups. University of Washington Reflection Groups graduate course. pdf
(1/18/2017) Math 308 Wiki Project Discussion. University of Washington informal talk. pdf
(11/3/2016) Cyclic sieving and Springer's regular elements. University of Minnesota Student-Run Combinatorics Seminar. web pdf
(10/6/2016) Symmetric group characters as symmetric functions (two talks). University of Washington CAT Seminar. pdf
(4/7/2016) $n!$ Conjecture Seminar (six talks). University of Washington $n!$ Seminar. pdf
(2/11/2016) On eigenvalue of representations of reflection groups and wreath products (two talks). University of Washington CAT Seminar. pdf
(1/12/2016) Three examples of algebraic geometry in algebraic combinatorics. University of Washington 123 Seminar. pdf
(12/9/2015) The fundamental theorem of Galois theory. University of Washington gradudate Algebra course. pdf
(11/5/2015) Introduction to cyclic sieving. University of Washington CAT seminar. pdf
(10/1/2015) Diagonal coinvariants and Tesler matrices. University of Washington Humphreys reading seminar. pdf
(7/13/2015) The PBW Theorem. University of Washington Humphreys reading seminar. pdf
(6/7/2014) Duality between Quasi-Symmetric Functions and the Solomon Descent Algebra. University of Washington graduate combinatorics course. pdf
(5/30/2014) Serre spectral sequence. University of Washington graduate group cohomology course. pdf

Mathematics notes

I've taken various notes for courses, seminar series, qualifying exams, etc. Some are well-edited, and some are not.

(2024) On the Stanley--Stembridge conjecture, 1 pages. Computationally verifies the key equation in a recent preprint of Tatsuyuki Hikita on the Stanley--Stembridge conjecture. ipynb
(2017) The Hilbert scheme of points in the plane is connected, 3 pages. Project for the Applied Algebraic Geometry course at the University of Washington in Spring 2017. pdf mp4
(2016) Combinatorics and Geometry of Polytopes, 64 pages. Notes for a graduate course on polytopes taught by Isabella Novik at the University of Washington in Spring 2016. Includes material on basics (e.g. equivalence of convex hull and intersection definitions), f and h numbers, duality, face lattices, simple and simplicial polytopes, the upper bound theorem, shellability, simplicial complexes, the Kruska--Katona theorem, the $g$-theorem, the lower bound theorem, rigidity and frameworks, matroid polytopes. The first half has been edited, though the second half has not and likely has typos. pdf zip
(2016) Introduction to Algebraic Geometry, 101 pages. Notes for a graduate course on algebraic geometry taught by S\'andor Kov\'acs at the University of Washington in Winter and Spring 2016. Includes material on classical algebraic geometry, sheaves, rational maps, blow ups, singularities, normality, Serre's conditions, finite morphisms, constructable sets, Zariski's main theorem, ringed spaces, coherent sheaves, schemes, fibers, dimension, divisors, invertible sheaves, differentials. The first portion has been edited, though later lectures have not been and likely have typos. pdf zip
(2016) Background for the $n!$ theorem, 24 pages. Lecture notes for an informal graduate seminar on Haiman's 2004 proof of the $n!$ conjecture. Discusses classical symmetric function theory, Kostka--Foulkes polynomials, Springer fibers, Hall--Littlewood symmetric functions, Macdonald symmetric functions, Garsia--Haiman modules, $k$-Schur functions, diagonal coinvariants, and sketches some of the broad outlines of Haiman's proof. pdf
(2015) Algebraic Number Theory, 62 pages. Notes for a graduate course on algebraic number theory taught by Bianca Viray at the University of Washington in Fall 2015. Includes material on rings of integers, Dedekind domains, fractional ideals, ramification, decomposition and inertia groups, cyclotomic fields, Gauss' reciprocity law, class groups, Minkowski's theorem, lattices, Dirichlet's unit group theorem, p-adics, the archimidean property, Ostrowski's theorems, valuations, Hensel's lemma, local fields, Newton polygons, ramification group, topological and profinite groups, induction, group cohomology, inflation and restriction. It is largely unedited. pdf zip
(2015) Humphreys section 23.3, 7 pages. An expanded account of the part of Humphreys' "Introduction to Lie Algebras and Representation Theory" proving Harish--Chandra's theorem. pdf
(2015) Bigrassmannians, pages. Research notes on characterizing (bi)grassmannian permutations in types A and B via pattern avoidance, plus their enumeration. Includes type D conjecture. Unpublished work from the beginning of my graduate career. pdf
(2014) Algebraic Combinatorics, 93 pages. Notes for a graduate course on algebraic combinatorics taught by Sara Billey at the University of Washington in Spring 2014. Includes material on symmetric group representations, the Pieri rule, symmetric functions, coalgebras, Sweedler notation, bialgebras, Hopf algebras, antipodes, homological properties of Hopf algebras, duality, and student lecturs on various papers. pdf zip
(2014) Group Cohomology, 61 pages. Notes for a graduate course on group cohomology taught by Julia Pevtsova at the University of Washington in Spring 2014. Includes material on derived functors, associated graded objects, spectral sequences, double complexes, the universal coefficient theorem, Cartan--Eilenberg resolutions, hyper-derived functors, Grothendieck spectral sequence, Lyndon--Hochschild--Serre spectral sequence, group cohomology rings, restriction, induction, corestriction, Frobenius reciprocity, G-invariants, Leray--Serre spectral sequence, the Gysin sequence, cohomology of homogeneous spaces, Eilenberg--MacLane spaces, finite generation, \v{C}ech and sheaf cohomology. pdf zip
(2014) Advanced Commutative Algebra, 53 pages. Notes for a graduate course on advanced commutative algebra taught by S. Paul Smith at the University of Washington in Fall 2014. Includes material on tensor-hom adjunction, flatness, support, primes, duality conditions, injectives, local cohomology, Matlis duality, injective resolutions, depth, projective dimension, Auslander--Buchsbaum formula, Rees' theorem, regular sequences, Krull dimension, principal ideal theorems, Krull's intersection theorem, Hilbert series, Gelfand--Kirillov dimension, associated graded rings, regular local rings, minimal resolutions, global dimension, Cohen--Macaulay rings. pdf zip
(2014) Algebraic Groups, 55 pages. Notes for a graduate course on algebraic groups taught by Julia Pevtsova at the University of Washington in Fall 2014. Includes material on affine group schemes, Hopf algebras, Weil restriction, \'etale algebras, Cartier duality, comodules, representations, characters, induction, connected components, K\"ahler differentials, Lie algebras, algebraic groups. pdf zip
(2014) Lie Groups and Representation Theory, 71 pages. Notes for a graduate course on compact Lie groups and their representation theory taught by Robin Graham at the University of Washington in Fall 2014. Includes material on Lie algebras, semisimplicity, Cartan's criterion, real forms, complexification, reductive Lie algebras, signature, quaternions, classifications, classical Lie groups, Cartan decomposition, Haar measure, representations, intertwining operators, the exponential map, complete reducibility, spherical harmonics, the Peter--Weyl theorem. Not well edited--beware of typos. pdf zip
(2013) Complex prelim notes, 16 pages. A summary of the key results used in the University of Washington Complex Analysis graduate preliminary exam as of 2013. Includes material on basics (e.g. Schwarz' lemma, open mapping theorem), integral formulas (Cauchy's, Schwarz', Jensen's), analytic extensions, root finding, uniform approximations, normal families, harmonic and subharmonic functions, inequalities, series, products, analytic continuation, residues, and conformal maps. pdf
(2013) Spring 2013 algebra notes, 16 pages. A summary of a graduate course at the University of Washington taught by S. Paul Smith. Topics included Noether normalization, a classical introduction to varieties, Ext, chain complexes, adjoint functors, homological dimensions, tensor products, exactness, Tor, Dedekind domains. pdf

Courses Taught

(2024 FA) Math 505a: Applied Probability, USC. On Brightspace.
(2024 SP) Math 407: Probability Theory, USC. On Blackboard.
(2023 FA) Math 425a: Fundamental Concepts of Analysis, USC. On Blackboard.
(2023 FA) Math 407: Probability Theory, USC. On Blackboard.
(2023 SP) Math 407: Probability Theory, USC. On Blackboard.
(2022 FA) Math 407: Probability Theory, USC. On Blackboard.
(2022 SP) Math 126g: Calculus II, USC. On Blackboard.
(2021 FA) Math 225: Linear Algebra and Differential Equations, USC. On Blackboard.
(2021 WI) Math 11: Introductory Probability and Statistics, UCSD. Course site.
(2021 WI) Math 20C: Calculus III, UCSD. Course site.
(2020 FA) Math 20C: Calculus III, UCSD. Course site.
(2020 FA) Math 109: Mathematical Reasoning, UCSD. Course site.
(2020 SP) Math 184: Enumerative Combinatorics, UCSD. Course site.
(2020 WI) Math 15A: Introduction to Discrete Mathematics, UCSD. Course site.
(2020 WI) Math 109: Mathematical Reasoning, UCSD. Course site.
(2019 FA) Math 20C: Calculus III, UCSD. Course site.
(2019 SP) Math 184A: Combinatorics, UCSD. Course site.
(2019 SP) Math 11: Introductory Probability and Statistics, UCSD. Course site.
(2019 WI) Math 20C: Calculus III, UCSD. Course site.
(2018 FA) Math 109: Mathematical Reasoning, UCSD. On TritonEd.
(2018 SP) Math 308: Matrix Algebra with Applications, University of Washington. Course site.
(2017 SU) Math 308: Matrix Algebra with Applications, University of Washington. Course site.
(2017 SP) Math 307: Introduction to Differential Equations, University of Washington. Course site.
(2016 FA) Math 308: Matrix Algebra with Applications, University of Washington. Course site.
(2015 SU) Math 307: Introduction to Differential Equations, University of Washington. Course site.
(2015 SP) Math 308: Matrix Algebra with Applications, University of Washington. Course site.
(2013 SU) Math 126: Calculus with Analytic Geometry III, University of Washington. Course site.

Contact

About me

Publications and preprints

Awards

In progress

Invited talks

Informal talks

Mathematics notes

Courses Taught

Courses TA'd