nLab string 2-group



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String theory



The string 2-group is a smooth 2-group-refinement of the topological group called the string group. It is the ∞-group extension induced by the smooth/stacky version of the first fractional Pontryagin class/second Chern class.


A string 2-group extension String(G)String(G) is defined for every simple simply connected compact Lie group GG, such as the spin group G=Spin(n)G = Spin(n) or the special unitary group G=SU(n)G = SU(n) (for non-low nn).

Since string structures arise predominantly as higher analogs of spin structures, the default choice is G=SpinG = Spin and in that case one usually just writes String=String(Spin)String = String(Spin), for short.

Recall first that the string group in Top is one step in the Whitehead tower of the orthogonal group.


For nn \in \mathbb{N} let Spin(n)Spin(n) denote the spin group, regarded as a topological group. Write BSpin(n)B Spin(n) \in Top for its classifying space and

12p 1:BSpin(n)B 4 \frac{1}{2}p_1 : B Spin(n) \to B^4 \mathbb{Z}

for a representative of the characteristic class called the first fractional Pontryagin class. Its homotopy fiber in the (∞,1)-topos Top \simeq ∞Grpd is denoted BString(n):=BO(n)7B String(n) := B O(n)\langle 7 \rangle

BString(n) * BSpin(n) 12p 1 B 4. \array{ B String(n) &\to& * \\ \downarrow && \downarrow \\ B Spin(n) &\stackrel{\frac{1}{2} p_1}{\to}& B^4 \mathbb{Z} } \,.

The loop space

String(n):=ΩBString(n) String(n) := \Omega B String(n)

is the ∞-group-object in Top called the string group.

Write now

(ΠDiscΓcoDisc):SmoothGrpdcoDiscΓDiscΠGrpdTop (\Pi \dashv Disc \dashv \Gamma \dashv coDisc) : Smooth\infty Grpd \stackrel{\overset{\Pi}{\to}}{\stackrel{\overset{Disc}{\leftarrow}}{\stackrel{\overset{\Gamma}{\to}}{\underset{coDisc}{\leftarrow}}}} \infty Grpd \simeq Top

for the (∞,1)-topos Smooth∞Grpd of smooth ∞-groupoids, regarded as a cohesive (∞,1)-topos over ∞Grpd.


There is a lift through Π\Pi of 12p 1\frac{1}{2} p_1 to the smooth first fractional Pontryagin class

12p 1:BSpin(n)B 3U(1) \frac{1}{2}\mathbf{p}_1 : \mathbf{B}Spin(n) \to \mathbf{B}^3 U(1)

in Smooth∞Grpd, where

This is shown in (FSS).


Write BString(n)\mathbf{B}String(n) for the homotopy fiber of the smooth first fractional Pontryagin class

BString * BSpin 12p 1 B 3U(1) \array{ \mathbf{B}String &\to& * \\ \downarrow && \downarrow \\ \mathbf{B}Spin &\stackrel{\frac{1}{2}\mathbf{p}_1}{\to}& \mathbf{B}^3 U(1) }

in Smooth∞Grpd. Its loop space object

String(n):=ΩBString(n) String(n) := \Omega \mathbf{B}String(n)

is the smooth ∞-group called the smooth string 2-group.



||:=|Π()|:SmoothGrpdΠGrpd||Top \vert - \vert := \vert\Pi(-)\vert : Smooth \infty Grpd \stackrel{\Pi}{\to} \infty Grpd \stackrel{\vert - \vert}{\to} Top

for the intrinsic geometric realization in Smooth∞Grpd.


The smooth string 2-group, def. , indeed maps under ||\vert-\vert to the topological string group:

|BString(n)|BString(n). \vert \mathbf{B}String(n) \vert \simeq B String(n) \,.

Since B 3U(1)\mathbf{B}^3 U(1) is presented by a simplicial presheaf that is degreewise presented by a paracompact smooth manifold (a finite product of the circle group with itself), it follows from the general properties of Π\Pi discussed at Smooth∞Grpd that Π\Pi preserves the homotopy fiber of 12p 1\frac{1}{2}\mathbf{p}_1.


Several explicit presentations of the string Lie 2-group are known.

By Lie integration of the string Lie 2-algebra

We discuss a presentation of the smooth string 2-group by Lie integration of the skeletal version of the string Lie 2-algebra.

Recall the identification of L-∞ algebras 𝔤\mathfrak{g} with their dual Chevalley-Eilenberg algebras CE(𝔤)CE(\mathfrak{g}).



μ:=,[,]:𝔰𝔬(n)b 2 \mu := \langle - ,[-,-]\rangle : \mathfrak{so}(n) \to b^2 \mathbb{R}

for the canonical degree-3 cocycle in the Lie algebra cohomology of the special orthogonal group, normalized such that the 3-form

Ω (Spin(n))CE(𝔰𝔬(n))μCE(b 2) \Omega^\bullet(Spin(n)) \hookleftarrow CE(\mathfrak{so}(n)) \stackrel{\mu}{\leftarrow} CE(b^2 \mathbb{R})

represents the image in de Rham cohomology of a generators of the integral cohomology group H 3(G,)H^3(G,\mathbb{Z}) \simeq \mathbb{Z}.

Define the string Lie 2-algebra

𝔰𝔱𝔯𝔦𝔫𝔤(n):=𝔰𝔬(n) μ \mathfrak{string}(n) := \mathfrak{so}(n)_\mu

to be given by the Chevalley-Eilenberg algebra

CE(𝔰𝔱𝔯𝔦𝔫𝔤(n)):= (𝔰𝔬(n) *b,d 𝔰𝔱𝔯𝔦𝔫𝔤) CE(\mathfrak{string}(n)) := \wedge^\bullet ( \mathfrak{so}(n)^* \oplus \langle b\rangle , d_{\mathfrak{string}})

which is that of 𝔰𝔬(n)\mathfrak{so}(n) with a single generator bb in degree 3 adjoined and the differential given by

d 𝔰𝔱𝔯𝔦𝔫𝔤| 𝔰𝔬(n) *=d 𝔰𝔬(n); d_{\mathfrak{string}}|_{\mathfrak{so}(n)^*} = d_{\mathfrak{so}(n)};
d 𝔰𝔱𝔯𝔦𝔫𝔤:bμ. d_{\mathfrak{string}} : b \mapsto \mu \,.

We have a pullback square in L AlgL_\infty Alg

𝔰𝔱𝔯𝔦𝔫𝔤(n) eb 𝔰𝔬(n) μ b 2. \array{ \mathfrak{string}(n) &\to& e b \mathbb{R} \\ \downarrow && \downarrow \\ \mathfrak{so}(n) &\stackrel{\mu}{\to}& b^2 \mathbb{R} } \,.

See string Lie 2-algebra for more discussion.


The Lie integration of 𝔰𝔱𝔯𝔦𝔫𝔤(n)\mathfrak{string}(n) yields a presentation of the smooth String 2-group, def.

cosk 3exp(𝔰𝔱𝔯𝔦𝔫𝔤(n))BString(n). \mathbf{cosk}_3 \exp(\mathfrak{string}(n)) \simeq \mathbf{B} String(n) \,.

This is essentially the model considered in (Henriques), discussed here in the context of Smooth∞Grpd as described in (FSS).


We observe the image under Lie integration of the L L_\infty-algebra pullback diagram from prop. is a pullback diagram in [CartSp smooth op,sSet] proj[CartSp_{smooth}^{op}, sSet]_{proj} that presents the defining homotopy fiber. Before applying the coskeleton operation we have immediately

exp():(𝔰𝔱𝔯𝔦𝔫𝔤(n) eb 𝔰𝔬(n) μ b 2)(exp(𝔰𝔱𝔯𝔦𝔫𝔤(n)) exp(eb) exp(𝔰𝔬(n)) μ exp(b 2)) \exp(-) \; :\; \left( \array{ \mathfrak{string}(n) &\to& e b \mathbb{R} \\ \downarrow && \downarrow \\ \mathfrak{so}(n) &\stackrel{\mu}{\to}& b^2 \mathbb{R} } \right) \;\mapsto \; \left( \array{ \exp(\mathfrak{string}(n)) &\to& \exp(e b \mathbb{R}) \\ \downarrow && \downarrow \\ \exp(\mathfrak{so}(n)) &\stackrel{\mu}{\to}& \exp(b^2 \mathbb{R}) } \right)

such that on the right we still have a pullback diagram.

We discuss the descent o this pullback diagram along the projection exp(𝔰𝔬(n))cosk 3exp(𝔰𝔬(n))\exp(\mathfrak{so}(n)) \to \mathbf{cosk}_3 \exp(\mathfrak{so}(n)).

Notice from Lie integration the weak equivalence

Δ :exp(b n)B n+1 c. \int_{\Delta^\bullet} : \exp(b^n \mathbb{R}) \simeq \mathbf{B}^{n+1}\mathbb{R}_c \,.

Let II be the set of maps Δ[4]exp(b 2)\partial \Delta[4] \to \exp(b^2 \mathbb{R}) that fit into a diagram

Δ[4] exp(b 2) Δ B 3 c Δ[4] B 3() c \array{ \partial \Delta[4] &\to& \exp(b^2 \mathbb{R}) \\ \downarrow && \downarrow^{\mathrlap{\int_{\Delta^\bullet}}} \\ && \mathbf{B}^3 \mathbb{R}_c \\ \downarrow && \downarrow \\ \Delta[4] &\to& \mathbf{B}^3 (\mathbb{Z} \to \mathbb{R})_c }

(closed 3-forms on 3-balls whose integral is an integer).


exp(b 2/):=cosk 3((I×Δ[4]) I×Δ[4]cosk 3exp(b 2)) \exp(b^2 \mathbb{R}/\mathbb{Z}) := \mathbf{cosk}_3 \left( (I \times \Delta[4])\coprod_{I \times \partial \Delta[4]} \mathbf{cosk_3} \exp(b^2 \mathbb{R}) \right)

for the result of filling all these by 4-cells. Similarly define exp(eb/)\exp(e b \mathbb{R}/\mathbb{Z}).

Then applying the coskeleton functor to the above pullback diagram and using the projection (FSS)

exp(𝔰𝔬(n)) exp(μ) exp(b 2) cosk 3exp(𝔰𝔬(n)) 12p 1 exp(b 2/) \array{ \exp(\mathfrak{so}(n)) &\stackrel{\exp(\mu)}{\to}& \exp(b^2 \mathbb{R}) \\ \downarrow && \downarrow \\ \mathbf{cosk}_3\exp(\mathfrak{so}(n)) &\stackrel{\frac{1}{2}\mathbf{p}_1}{\to}& \exp(b^2 \mathbb{R}/\mathbb{Z}) }

we get the diagram

cosk 3(𝔰𝔱𝔯𝔦𝔫𝔤(n)) exp(eb/) cosk 3exp(𝔰𝔬) 12p 1 exp(b 2/). \array{ \mathbf{cosk}_3 (\mathfrak{string}(n)) &\to& \exp(e b \mathbb{R}/\mathbb{Z}) \\ \downarrow && \downarrow \\ \mathbf{cosk}_3 \exp(\mathfrak{so}) &\stackrel{\frac{1}{2}\mathbf{p}_1}{\to}& \exp(b^2 \mathbb{R}/\mathbb{Z}) } \,.

This is again a pullback diagram of a fibration resolution of the point inclusion, hence presents the homotopy fiber in question.

By strict Lie 22-group

A realization of the string 2-group as a strict 2-group internal to diffeological spaces was given in (BCSS).

This is one of three different (there should be more), weakly equivalent such strict 2-group internal to diffeological space models that are discussed in the (to date unpublished)

(This particular section, and its results, are joint work of Urs Schreiber and Danny Stevenson).

We have the following pattern of routes through Lie integration:

StrLieωGrpd StrLieωGrpd LieCrsdCmplx Π nSCE exp() L Algebras StrL Algebras DiffCrsdCmplx \array{ StrLie \omega Grpd &&&& StrLie \omega Grpd &\stackrel{\simeq}{\leftarrow}& LieCrsdCmplx \\ \uparrow^{\Pi_n S CE} &&&& \uparrow && \uparrow^{\exp(-)} \\ L_\infty Algebras && \leftarrow&& Str L_\infty Algebras &\to& DiffCrsdCmplx }

Here StrLieωGrpdStrLie \omega Grpd is strict omega-groupoids internal to diffeological spaces, LieCrsCmplxLieCrsCmplx is accordingly smooth crossed complexes , L AlgebraL_\infty Algebra is all L-infinity algebras and StrL AlgebraStr L_\infty Algebra is strict L L_\infty-algebras. The vertical morphism on the right is term-wise ordinary Lie integration. The other vertical morphisms take an L-infinity algebra, form the sheaf on Diff of flat ∞-Lie algebroid differential forms, and then take path n-groupoid Π n()\Pi_n(-) of that.

For the String-case this yields

Π 2(Ω fl (,𝔰𝔬 μ 3)) BString Mick BString BCSS | (Ω^SpinPSpin) 𝔰𝔬 μ 3 𝔰𝔱𝔯𝔦𝔫𝔤 (Ω^𝔰𝔬P𝔰𝔬), \array{ \Pi_2(\Omega^\bullet_{fl}(-,\mathfrak{so}_{\mu_3})) &\stackrel{\simeq}{\mapsto}& \mathbf{B} String_{Mick} &\stackrel{\simeq}{\mapsto}& \mathbf{B} String_{BCSS} &\leftarrow|& (\hat \Omega Spin \to P Spin) \\ \uparrow &&&\nearrow& \uparrow && \uparrow \\ \mathfrak{so}_{\mu_3} &&\stackrel{\simeq}{\mapsto}&& \mathfrak{string} &\mapsto& (\hat \Omega \mathfrak{so} \to P \mathfrak{so}) } \,,


  • 𝔰𝔬 μ 3\mathfrak{so}_{\mu_3} denotes the weak, skeletal String Lie 2-algebra

  • 𝔰𝔱𝔯𝔦𝔫𝔤\mathfrak{string} its equivalent strict version given by BCSS

  • the diagonal morphism is the construction in BCSS.

  • the strict 2-groupoid Π 2(Ω fl (,𝔤 μ 3))\Pi_2(\Omega^\bullet_{fl}(-,\mathfrak{g}_{\mu_3})) has, notice, as morphism smooth paths in Spin(n)Spin(n) that are composed by concatenation

  • the 2-groupoid BString Mick\mathbf{B}String_{Mick} is a version of the String Lie 2-group that manifestly uses the Mickelsson cocycle? (morphism are paths in Spin(n)Spin(n) that are composed using the group product)

  • the 2-groupoid BString BCSS\mathbf{B}String_{BCSS} is the version given in BCSS (morhisms again are paths in Spin(n)Spin(n) that are composed using the group product).

As a finite-dimensional weak Lie 2-group


As an automorphism 2-group of fermionic CFT

The string 2-group also appears as a certain automorphism 2-group inside the 3-category of fermionic conformal nets (Douglas-Henriques)

As the automorphisms of the Wess-Zumino-Witten gerbe 2-connection

For GG a compact simply connected simple Lie group, there is the “WZW gerbe”, hence the circle 2-bundle with connection on GG whose curvature 3-form is the left invariant extension θ[θθ]\langle \theta \wedge [\theta \wedge \theta]\rangle of the canonical Lie algebra 3-cocycle to the group

WZW:GB 2U(1). \mathcal{L}_{WZW} \;\colon\; G \longrightarrow \mathbf{B}^2U(1) \,.

The string 2-group is the smooth 2-group of automorphism of WZW\mathcal{L}_{WZW} which cover the left action of GG on itself (hence the “Heisenberg 2-group” of WZW\mathcal{L}_{WZW} regarded as a prequantum 2-bundle)

Aut( WZW)String(G), \mathbf{Aut}(\mathcal{L}_{WZW}) \simeq String(G) \,,

This is due to (Fiorenza-Rogers-Schreiber 13, section 2.6.1).

fivebrane 6-group\to string 2-group \to spin group \to special orthogonal group \to orthogonal group \hookrightarrow general linear group


A crossed module presentation of a topological realization of the string 2-group is implicit in

A realization of the string 2-group in ∞-groupoids internal to Banach spaces by Lie integration of the skeletal version of the string Lie 2-algebra is in

A realization of the string 2-group in strict 2-groups internal to Frechet manifolds by Lie integration of a strict Lie 2-algebra incarnation of the string Lie 2-algebra in in

A realization of the string 2-group as a 2-group in finite-dimensional smooth manifolds in in

A discussion as an ∞-group object in Smooth∞Grpd and the realization of the smooth first fractional Pontryagin class is in

and in section 4.1 of

A 2-group model which has a smoothening of the topological string group in lowest degree has been given in

A construction explicitly in terms of the “basicbundle gerbe on GG:

Via fermionic nets/2-Clifford algebra:

The realization of the string 2-group as the Heisenberg 2-group of the WZW gerbe is due to

A model of the string 2-group using the smooth free loop space (instead of the based loop space) is dicussed in

Discussion in the context of matrix factorizations and equivariant K-theory:

Further on 2-group-extensions by the circle 2-group:

of tori:

of finite subgroups of SU(2) (to Platonic 2-groups):

Discussion of a general definition of smooth string group extensions AString(H)HA\rightarrow \mathrm{String}(H)\rightarrow H of a compact simply connected Lie group HH, with AA not necessarily chosen to be BU(1)\mathbf{B}U(1) but only of the same homotopy type, in

Last revised on January 11, 2024 at 12:43:48. See the history of this page for a list of all contributions to it.