REVIEW
Expanding Lie (super)algebras through abelian semigroups
High Energy Physics - Theory · Submitted 21 Jun 2006 · 36 references
Pith's verdict: A clean generalization of Maurer–Cartan expansion whose real payoff is the invariant-tensor construction in section VII; the Z4 'new algebra' claim holds but needs one paragraph of argument the paper doesn't give.
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Referee report
The paper
The paper claims
Multiplying a Lie algebra by an abelian semigroup produces another Lie algebra, and the semigroup's multiplication table alone controls which subalgebras, reductions, and invariant tensors come out.
The reading
What carries the argument
The $n$-selector. For semigroup elements $a_1, ..., a_n$ it equals 1 when their product is a given element and 0 otherwise; the two-selector version is just the multiplication table written as a matrix. This single object does three jobs at once: it multiplies structure constants in the expanded bracket, it decides which products of subsets stay inside which other subsets (the 'resonance condition' $S_p S_q$ contained in the intersection of the $S_r$ required by $[V_p, V_q]$), and it slots into the invariant-tensor formula $<T_{(A_1,a_1)} ... T_{(A_n,a_n)}> = c_c K^{c}_{a_1 ... a_n} <T_{A_1} ... T_{A_n}>$. The last formula is the paper's practical contribution: it manufactures nonvanishing invariant tensors that the ordinary supertrace cannot supply, because in the $S_E$ case the supertrace collapses to a single component.
Extended reading notes
Core claim
The central claim is Theorem III.1: if $S$ is an abelian semigroup with two-selector $K^{~}_{}$, i.e. the array that records which product of two semigroup elements equals which third, and $g$ is a Lie (super)algebra with structure constants $C^{C}_{AB}$, then the bracket $[T_{(A,a)}, T_{(B,b)}] = K^{c}_{ab} C^{C}_{AB} T_{(C,c)}$ closes and satisfies Jacobi. Jacobi holds because the selectors are associative and symmetric, so the semigroup never disturbs the algebra's own identity. Everything else in the paper is built on that: a 'resonant' partition of $S$ into subsets whose products land inside the subsets dictated by the grading of $g$ gives a subalgebra of the expanded algebra (Theorem IV.2); a splitting of those subsets into a kept part and a discarded part gives a reduced algebra (Theorem VI.1); and the $n$-selector, the obvious generalization of the two-selector, gives invariant tensors for both the expanded algebra and its reduction (Theorems VII.1 and VII.2). The authors then show that the Maurer–Cartan expansion of de Azcárraga and collaborators is exactly the case where $S$ is the truncated power semigroup $S_E$ followed by setting the zero element's generators to zero, so the older method is a special case rather than a rival.
Load-bearing premise
The claim that different semigroup choices give genuinely new algebras rests on the observation that $Z_4$ is not the truncated power semigroup $S_E$; the paper does not check isomorphism classes, so 'new' here means 'not produced by the earlier procedure as defined' rather than 'provably non-isomorphic to every expanded algebra'.
Editorial extensions
If this is right
- The Maurer–Cartan power-series expansion of de Azcárraga et al. is recovered in full, including the truncation conditions, as a zero-element reduction of an $S_E$-expansion, so the older results are subsumed rather than replaced.
- The M algebra, with its 583 bosonic generators, is obtained from $osp(32|1)$ by an $S_E$-expansion with truncation order 2 followed by a reduction; the D'Auria–Fré-like superalgebra with an extra fermionic generator follows from order 3.
- Choosing the cyclic group $Z_4$ instead of $S_E$ gives a superalgebra with two fermionic charges obeying the same anticommutator and two sets of 'AdS-boost' generators sharing one Lorentz algebra, which the Maurer–Cartan procedure as defined does not produce.
- The five-brane superalgebra is recovered as a reduced algebra of a resonant subalgebra, and the generalized Iñönü–Wigner contraction is identified as a particular reduction rather than a subalgebra.
- Because invariant tensors are available for resonant subalgebras and their zero-element reductions, Chern–Simons and transgression Lagrangians can be written for these algebras in odd dimensions, which is the stated route to an eleven-dimensional supergravity Lagrangian for the M algebra.
Reading between the lines
- The construction suggests a general classification programme the authors only gesture at: enumerate finite abelian semigroups up to some small order, compute their resonant partitions against a fixed grading of $osp(32|1)$, and you get a finite catalogue of eleven-dimensional superalgebras with known invariant tensors. That is a concrete, checkable enumeration rather than an open-ended search.
- The reduction theorems look like a clean algebraic home for contraction and deformation. If every generalized Iñönü–Wigner contraction is a reduction of a resonant subalgebra, then the space of algebras reachable from a fixed $g$ has a partial order by semigroup refinement, and questions about which contractions are deformable back become questions about that order.
- The invariant-tensor formula depends only on the $n$-selector, so it applies verbatim to any semigroup, including non-$S_E$ ones. A natural test is whether the $Z_4$ superalgebra's selector-built tensor gives a Chern–Simons action that is not equivalent to any $S_E$-derived one; if it is equivalent, the 'new algebra' claim weakens considerably.
- The paper's own remark that abelianity is essential while finiteness and discreteness might be dropped points at a continuum version: replace the semigroup by a commutative monoid of functions or a commutative algebra, and the selector becomes a kernel. Whether the Jacobi identity survives that limit is the obvious next question.
Original abstract
We propose an outgrowth of the expansion method introduced by de Azcarraga et al. [Nucl. Phys. B 662 (2003) 185]. The basic idea consists in considering the direct product between an abelian semigroup S and a Lie algebra g. General conditions under which relevant subalgebras can systematically be extracted from S \times g are given. We show how, for a particular choice of semigroup S, the known cases of expanded algebras can be reobtained, while new ones arise from different choices. Concrete examples, including the M algebra and a D'Auria-Fre-like Superalgebra, are considered. Finally, we find explicit, non-trace invariant tensors for these S-expanded algebras, which are essential ingredients in, e.g., the formulation of Supergravity theories in arbitrary space-time dimensions.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We propose an outgrowth of the expansion method introduced by de Azcarraga et al. [Nucl. Phys. B 662 (2003) 185]. The basic idea consists in considering the direct product between an abelian semigroup S and a Lie algebra g. General conditions under which relevant subalgebras can systematically be extracted from S \times g are given. We show how, for a particular choice of semigroup S, the known cases of expanded algebras can be reobtained, while new ones arise from different choices. Concrete examples, including the M algebra and a D'Auria-Fre-like Superalgebra, are considered. Finally, we find explicit, non-trace invariant tensors for these S-expanded algebras, which are essential ingredients in, e.g., the formulation of Supergravity theories in arbitrary space-time dimensions.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We propose an outgrowth of the expansion method introduced by de Azcarraga et al. [Nucl. Phys. B 662 (2003) 185]. | background 0.63 | 0.71 | 0.04 | asserted 0.95 |
| s1 | The basic idea consists in considering the direct product between an abelian semigroup S and a Lie algebra g. | method 0.90 | 0.31 | 0.06 | asserted 1.00 |
| s2 | General conditions under which relevant subalgebras can systematically be extracted from S \times g are given. | contribution 0.65 | 0.84 | 0.16 | asserted 1.00 |
| s3 | We show how, for a particular choice of semigroup S, the known cases of expanded algebras can be reobtained, while new ones arise from different choices. | result 0.77 | 0.93 | 0.29 | asserted 1.00 |
| s4 | Concrete examples, including the M algebra and a D'Auria-Fre-like Superalgebra, are considered. | contribution 0.81 | 0.81 | 0.07 | asserted 0.96 |
| s5 | Finally, we find explicit, non-trace invariant tensors for these S-expanded algebras, which are essential ingredients in, e.g., the formulation of Supergravity theories in arbitrary space-time dimensions. | result 0.92 | 0.95 | 0.41 | asserted 1.00 |
What was examined, and what was not
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Figures
Showing 4 of the paper's 8 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-th/0606215 by the authors.
References
How later papers used it
73 later papers cite it
2 later papers by other authors are listed below: 2 used its method or tool. 5 more papers by the same authors are listed: 5 used its method or tool.
Uses its method or tool 7 of 35 papers with this label · 5 by the same authors
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Combined this paper's S-expansion method with the Chern-Simons formalism to construct three-dimensional non-relativistic supergravity actions.
Non-Lorentzian Supergravity and Kinematical Superalgebras (2024) · 1.00 · paraphrased: the licence does not allow quoting
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Since the direct product of $S$ with a Lie algebra is again a Lie algebra [ref] , one can define the expanded Nappi-Witten algebra [ref] S×={ S⊗{ ,_a,} }. whose generators are obtained by considering the tensor product of the semigroup elements and the generators of the $$ algebra, i.e. ^(i)_a=λ_i ⊗_a , ^(i)=λ_i⊗ , ^(i) =λ_i ⊗ , The commutation relations of the expanded Nappi-Witten algebra are [ ^(i),^(j)_a ] = ϵ_a^ b ^(i♢j)_b , [ ^(i)_a,^(j)_b ] = -ϵ_ab^(i♢j) .
Non-relativistic spin-3 symmetries in 2+1 dimensions from expanded/extended Nappi-Witten algebras (2022) · Expansions of higher-spin Nappi-Witten algebras · 0.82
By the same authors
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Followed this paper's definitions to show that a partition satisfies relations and that a reduced algebra corresponds to it.
Maxwell Superalgebras and Abelian Semigroup Expansion (2014) (same authors) · 0.81 · paraphrased: the licence does not allow quoting
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Applied this paper's S-expansion method to construct a non-relativistic Maxwell-Bloch superalgebra and its non-degenerate invariant tensor.
Non-Relativistic Chern-Simons Supergravity with Torsion (2026) (same authors) · 0.97 · paraphrased: the licence does not allow quoting
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Followed this paper's definitions to consider the S-expansion of the Lie algebra so(4,2) using a finite abelian semigroup.
Standard General Relativity from Chern-Simons Gravity (2009) (same authors) · 0.99 · paraphrased: the licence does not allow quoting
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Obtained the three-dimensional AdS-Carroll algebra by means of this paper's S-expansion method.
3D Carrollian gravity from 2D Euclidean symmetry (2025) (same authors) · 0.99 · paraphrased: the licence does not allow quoting
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Obtained its new non-relativistic superalgebras from two relativistic N=2 AdS-L superalgebras considering this paper's semigroup expansion procedure.
Three-dimensional non-relativistic extended supergravity with cosmological constant (2020) (same authors) · 0.97 · paraphrased: the licence does not allow quoting
How the literature uses it
60 arXiv papers cite this in 242 sentences
Papers citing it · 138 sentences without a confident label
Builds on it 2 papers
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[ref] , can be generalized so as to obtain expanded higher-order Lie algebras.
S-Expansion of Higher-Order Lie Algebras · Comments · 0.93
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In this work, we present a supersymmetric extension of the three-dimensional exotic Newtonian CS gravity introduced in [ref] by applying the semigroup expansion ($S$-expansion) [ref] to the $𝒩=2$ AdS supergravity theory.
Three-dimensional exotic Newtonian supergravity theory with cosmological constant · Introduction · 0.87
Uses its method or tool 30 papers
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In the following sections, we will apply the $S$-expansion method [ref] to the super Nappi-Witten algebra in order to obtain different NR superalgebras.
Non-relativistic three-dimensional supergravity theories and semigroup expansion method · Nappi-Witten superalgebra and Chern-Simons action · 1.00
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By applying the S-expansion method [ref] , and inspired by the results obtained in [ref] , we construct the corresponding NR MB superalgebra together with its non-degenerate invariant tensor, which constitutes the essential ingredient for defining a well-defined CS supergravity action.
Non-Relativistic Chern-Simons Supergravity with Torsion · Non-relativistic regime of the Mielke-Baekler supergravity · 0.97
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It is the purpose of this Letter to show that the SSEP algebra $𝔰𝔬 ( D-1,1 ) ⊕𝔰𝔬 ( D-1,2 )$ can be obtained from the AdS algebra $𝔰𝔬 ( D-1,2 )$ via the $S$-expansion procedure with an appropriate semigroup $S$ [ref] .
A generalized action for $\left( 2 + 1 \right)$-dimensional Chern--Simons gravity · Introduction · 0.85
Compares with it 3 papers
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In this work we have found a relation between two procedures for the expansion of Lie algebras, namely a relation between the so-called `power series expansion method' of Ref. [ref] and the `$S$-expansion Procedure' of Ref. [ref] .
Dual Formulation of the Lie Algebra S-expansion Procedure · Comments and Possible Developments · 0.99
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The present work shows the difference between the possibilities of the $S$-expansion procedure [ref] (using semigroups) and the MC forms expansion (using a parameter).
Standard General Relativity from Chern-Simons Gravity · Comments and Possible Developments · 0.91
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In a sense, Galilean contraction procedures provide an opposite construction to semigroup expansions which also give rise to Galilean subalgebras [ref] .
Asymmetric Galilean conformal algebras · Discussion · 0.88
Cites it as background 18 papers
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In Refs. [ref] , [ref] , [ref] was proposed a natural outgrowth of power series expansion method, which is based on combining the structure constant of the algebra $𝔤$ with the inner law of a semigroup $S$ in order to define the Lie bracket of a new $S$-expanded algebra.
Einstein-Hilbert action with cosmological term from Chern-Simons gravity · Introduction · 0.87
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An alternative expansion procedure to the method of power series expansion is the $S$-expansion method [ref] .
Dual Formulation of the Lie Algebra S-expansion Procedure · Introduction · 0.80
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Some time ago was shown that the standard, five-dimensional General Relativity can be obtained from Chern-Simons gravity theory for a certain Lie algebra $𝔅$ [ref] , which was obtained from the AdS algebra and a particular semigroup $S$ by means of the S-expansion procedure introduced in Refs. [ref] .
Stellar equilibrium in Einstein-Chern-Simons gravity · Introduction · 0.95
Discussion
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