Definition from OPEs

Using operator product expansions (OPEs), an ${\displaystyle N}$-point function on the sphere can be written as a combination of three-point structure constants, and universal quantities called ${\displaystyle N}$-point conformal blocks.^[1][2]

Given an ${\displaystyle N}$-point function, there are several types of conformal blocks, depending on which OPEs are used. In the case ${\displaystyle N=4}$, there are three types of conformal blocks, corresponding to three possible decompositions of the same four-point function. Schematically, these decompositions read

${\displaystyle \left\langle V_{1}V_{2}V_{3}V_{4}\right\rangle =\sum _{s}C_{12s}C_{s34}{\mathcal {F}}_{s}^{\text{(s-channel)}}=\sum _{t}C_{14t}C_{t23}{\mathcal {F}}_{t}^{\text{(t-channel)}}=\sum _{u}C_{13u}C_{24u}{\mathcal {F}}_{u}^{\text{(u-channel)}}\ ,}$

where ${\displaystyle C}$ are structure constants and ${\displaystyle {\mathcal {F}}}$ are conformal blocks. The sums are over representations of the conformal algebra that appear in the CFT's spectrum. OPEs involve sums over the spectrum, i.e. over representations and over states in representations, but the sums over states are absorbed in the conformal blocks.

In two dimensions, the symmetry algebra factorizes into two copies of the Virasoro algebra, called left-moving and right-moving. If the fields are factorized too, then the conformal blocks factorize as well, and the factors are called Virasoro conformal blocks. Left-moving Virasoro conformal blocks are locally holomorphic functions of the fields' positions ${\displaystyle z_{i}}$; right-moving Virasoro conformal blocks are the same functions of ${\displaystyle {\bar {z}}_{i}}$. The factorization of a conformal block into Virasoro conformal blocks is of the type

${\displaystyle {\mathcal {F}}_{s_{L}\otimes s_{R}}^{\text{(s-channel)}}(\{z_{i}\})={\mathcal {F}}_{s_{L}}^{\text{(s-channel, Virasoro)}}(\{z_{i}\}){\mathcal {F}}_{s_{R}}^{\text{(s-channel, Virasoro)}}(\{{\bar {z}}_{i}\})\ ,}$

where ${\displaystyle s_{L},s_{R}}$ are representations of the left- and right-moving Virasoro algebras respectively.

Definition from Virasoro Ward identities

Conformal Ward identities are the linear equations that correlation functions obey, as a result of conformal symmetry.

In two dimensions, conformal Ward identities decompose into left-moving and right-moving Virasoro Ward identities. Virasoro conformal blocks are solutions of the Virasoro Ward identities.^[3][4]

OPEs define specific bases of Virasoro conformal blocks, such as the s-channel basis in the case of four-point blocks. The blocks that are defined from OPEs are special cases of the blocks that are defined from Ward identities.

Linear equations

From the corresponding correlation function, conformal blocks inherit linear equations: global and local Ward identities, and BPZ equations if at least one field is degenerate.^[2]

In particular, in an ${\displaystyle N}$-point block on the sphere, global Ward identities reduce the dependence on the ${\displaystyle N}$ field positions to a dependence on ${\displaystyle N-3}$ cross-ratios. In the case ${\displaystyle N=4,}$

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|\{z_{i}\})=z_{23}^{\Delta _{1}-\Delta _{2}-\Delta _{3}+\Delta _{4}}z_{13}^{-2\Delta _{1}}z_{34}^{\Delta _{1}+\Delta _{2}-\Delta _{3}-\Delta _{4}}z_{24}^{-\Delta _{1}-\Delta _{2}+\Delta _{3}-\Delta _{4}}{\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|z),}$

where ${\displaystyle z_{ij}=z_{i}-z_{j},}$ and

${\displaystyle z={\frac {z_{12}z_{34}}{z_{13}z_{24}}}}$

is the cross-ratio, and the reduced block ${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|z)}$ coincides with the original block where three positions are sent to ${\displaystyle (0,\infty ,1),}$

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|z)={\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|z,0,\infty ,1).}$

Singularities

Like correlation functions, conformal blocks are singular when two fields coincide. Unlike correlation functions, conformal blocks have very simple behaviours at some of these singularities. As a consequence of their definition from OPEs, s-channel four-point blocks obey

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|z){\underset {z\to 0}{=}}z^{\Delta _{s}-\Delta _{1}-\Delta _{2}}\left(1+\sum _{n=1}^{\infty }c_{n}z^{n}\right),}$

for some coefficients ${\displaystyle c_{n}.}$ On the other hand, s-channel blocks have complicated singular behaviours at ${\displaystyle z=1,\infty }$: it is t-channel blocks that are simple at ${\displaystyle z=1}$, and u-channel blocks that are simple at ${\displaystyle z=\infty .}$

In a four-point block that obeys a BPZ differential equation, ${\displaystyle z=0,1,\infty }$ are regular singular points of the differential equation, and ${\displaystyle \Delta _{s}-\Delta _{1}-\Delta _{2}}$ is a characteristic exponent of the differential equation. For a differential equation of order ${\displaystyle n}$, the ${\displaystyle n}$ characteristic exponents correspond to the ${\displaystyle n}$ values of ${\displaystyle \Delta _{s}}$ that are allowed by the fusion rules.

Field permutations

Permutations of the fields ${\displaystyle V_{i}(z_{i})}$ leave the correlation function

${\displaystyle \left\langle \prod _{i=1}^{N}V_{i}(z_{i})\right\rangle }$

invariant, and therefore relate different bases of conformal blocks with one another. In the case of four-point blocks, t-channel blocks are related to s-channel blocks by^[2]

${\displaystyle {\mathcal {F}}_{\Delta }^{(t)}(\Delta _{1},\Delta _{2},\Delta _{3},\Delta _{4}|z_{1},z_{2},z_{3},z_{4})={\mathcal {F}}_{\Delta }^{(s)}(\Delta _{1},\Delta _{4},\Delta _{3},\Delta _{2}|z_{1},z_{4},z_{3},z_{2}),}$

or equivalently

${\displaystyle {\mathcal {F}}_{\Delta }^{(t)}(\Delta _{1},\Delta _{2},\Delta _{3},\Delta _{4}|z)={\mathcal {F}}_{\Delta }^{(s)}(\Delta _{1},\Delta _{4},\Delta _{3},\Delta _{2}|1-z).}$

Fusing matrix

The change of bases from s-channel to t-channel four-point blocks is characterized by the fusing matrix (or fusion kernel) ${\displaystyle F}$, such that

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{(s)}(\{\Delta _{i}\}|\{z_{i}\})=\int _{i\mathbb {R} }dP_{t}\ F_{\Delta _{s},\Delta _{t}}{\begin{bmatrix}\Delta _{2}&\Delta _{3}\\\Delta _{1}&\Delta _{4}\end{bmatrix}}{\mathcal {F}}_{\Delta _{t}}^{(t)}(\{\Delta _{i}\}|\{z_{i}\}).}$

The fusing matrix is a function of the central charge and conformal dimensions, but it does not depend on the positions ${\displaystyle z_{i}.}$ The momentum ${\displaystyle P_{t}}$ is defined in terms of the dimension ${\displaystyle \Delta _{t}}$ by

${\displaystyle \Delta ={\frac {c-1}{24}}-P^{2}.}$

The values ${\displaystyle P\in i\mathbb {R} }$ correspond to the spectrum of Liouville theory.

We also need to introduce two parameters ${\displaystyle Q,b}$ related to the central charge ${\displaystyle c}$,

${\displaystyle c=1+6Q^{2},\qquad Q=b+b^{-1}.}$

Assuming ${\displaystyle c\notin (-\infty ,1)}$ and ${\displaystyle P_{i}\in i\mathbb {R} }$, the explicit expression of the fusing matrix is^[5]

${\displaystyle {\begin{aligned}F_{\Delta _{s},\Delta _{t}}&{\begin{bmatrix}\Delta _{2}&\Delta _{3}\\\Delta _{1}&\Delta _{4}\end{bmatrix}}=\\&=\left(\prod _{\pm }{\frac {\Gamma _{b}(Q\pm 2P_{s})}{\Gamma _{b}(\pm 2P_{t})}}\right){\frac {\Xi _{+}(P_{1},P_{4},P_{t})\Xi _{+}(P_{2},P_{3},P_{t})}{\Xi _{-}(P_{1},P_{2},P_{s})\Xi _{-}(P_{3},P_{4},P_{s})}}\times \\&\quad \times \int _{{\frac {Q}{4}}+i\mathbb {R} }du\ S_{b}\left(u-P_{12s}\right)S_{b}\left(u-P_{s34}\right)S_{b}\left(u-P_{23t}\right)S_{b}\left(u-P_{t14}\right)\\&\qquad \times S_{b}\left({\tfrac {Q}{2}}-u+P_{1234}\right)S_{b}\left({\tfrac {Q}{2}}-u+P_{st13}\right)S_{b}\left({\tfrac {Q}{2}}-u+P_{st24}\right)S_{b}\left({\tfrac {Q}{2}}-u\right)\end{aligned}}}$

where ${\displaystyle \Gamma _{b}}$ is a double gamma function,

${\displaystyle {\begin{aligned}S_{b}(x)&={\frac {\Gamma _{b}(x)}{\Gamma _{b}(Q-x)}}\\[6pt]\Xi _{\epsilon }(P_{1},P_{2},P_{3})&=\prod _{\underset {\epsilon _{1}\epsilon _{2}\epsilon _{3}=\epsilon }{\epsilon _{1},\epsilon _{2},\epsilon _{3}=\pm }}\Gamma _{b}\left({\tfrac {Q}{2}}+\sum _{i}\epsilon _{i}P_{i}\right)\\[6pt]P_{ijk}&=P_{i}+P_{j}+P_{k}\end{aligned}}}$

Although its expression is simpler in terms of momentums ${\displaystyle P_{i}}$ than in terms of conformal dimensions ${\displaystyle \Delta _{i}}$, the fusing matrix is really a function of ${\displaystyle \Delta _{i}}$, i.e. a function of ${\displaystyle P_{i}}$ that is invariant under ${\displaystyle P_{i}\to -P_{i}}$. In the expression for the fusing matrix, the integral is a hyperbolic Barnes integral. Up to normalization, the fusing matrix coincides with Ruijsenaars' hypergeometric function, with the arguments ${\displaystyle P_{s},P_{t}}$ and parameters ${\displaystyle b,b^{-1},P_{1},P_{2},P_{3},P_{4}}$.^[6] The fusing matrix has several different integral representations, and obeys many nontrivial identities.^[7]

In ${\displaystyle N}$-point blocks on the sphere, the change of bases between two sets of blocks that are defined from different sequences of OPEs can always be written in terms of the fusing matrix, and a simple matrix that describes the permutation of the first two fields in an s-channel block,^[3]

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{(s)}(\Delta _{1},\Delta _{2},\Delta _{3},\Delta _{4}|z_{1},z_{2},z_{3},z_{4})=e^{i\pi (\Delta _{s}-\Delta _{1}-\Delta _{2})}{\mathcal {F}}_{\Delta _{s}}^{(s)}(\Delta _{2},\Delta _{1},\Delta _{3},\Delta _{4}|z_{2},z_{1},z_{3},z_{4}).}$

From the definition

The definition from OPEs leads to an expression for an s-channel four-point conformal block as a sum over states in the s-channel representation, of the type ^[8]

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{\text{(s)}}(\{\Delta _{i}\}|z)=z^{\Delta _{s}-\Delta _{1}-\Delta _{2}}\sum _{L,L'}z^{|L|}f_{12s}^{L}Q_{L,L'}^{s}f_{43s}^{L'}\ .}$

The sums are over creation modes ${\displaystyle L,L'}$ of the Virasoro algebra, i.e. combinations of the type ${\displaystyle L=\prod _{i}L_{-n_{i}}}$ of Virasoro generators with ${\displaystyle 1\leq n_{1}\leq n_{2}\leq \cdots }$, whose level is ${\displaystyle |L|=\sum n_{i}}$. Such generators correspond to basis states in the Verma module with the conformal dimension ${\displaystyle \Delta _{s}}$. The coefficient ${\displaystyle f_{12s}^{L}}$ is a function of ${\displaystyle \Delta _{1},\Delta _{2},\Delta _{s},L}$, which is known explicitly. The matrix element ${\displaystyle Q_{L,L'}^{s}}$ is a function of ${\displaystyle c,\Delta _{s},L,L'}$ which vanishes if ${\displaystyle |L|\neq |L'|}$, and diverges for ${\displaystyle |L|=N}$ if there is a null vector at level ${\displaystyle N}$. Up to ${\displaystyle |L|=1}$, this reads

${\displaystyle {\mathcal {F}}_{\Delta _{s}}^{\text{(s)}}(\{\Delta _{i}\}|z)=z^{\Delta _{s}-\Delta _{1}-\Delta _{2}}{\Bigg \{}1+{\frac {(\Delta _{s}+\Delta _{1}-\Delta _{2})(\Delta _{s}+\Delta _{4}-\Delta _{3})}{2\Delta _{s}}}z+O(z^{2}){\Bigg \}}\ .}$

(In particular, ${\displaystyle Q_{L_{-1},L_{-1}}^{s}={\frac {1}{2\Delta _{s}}}}$ does not depend on the central charge ${\displaystyle c}$.)

Zamolodchikov's recursive representation

In Alexei Zamolodchikov's recursive representation of four-point blocks on the sphere, the cross-ratio ${\displaystyle z}$ appears via the nome

${\displaystyle q=\exp -\pi {\frac {F({\frac {1}{2}},{\frac {1}{2}},1,1-z)}{F({\frac {1}{2}},{\frac {1}{2}},1,z)}}{\underset {z\to 0}{=}}{\frac {z}{16}}+{\frac {z^{2}}{32}}+O(z^{3})\quad \iff \quad z={\frac {\theta _{2}(q)^{4}}{\theta _{3}(q)^{4}}}{\underset {q\to 0}{=}}16q-128q^{2}+O(q^{3})}$

where ${\displaystyle F}$ is the hypergeometric function, and we used the Jacobi theta functions

${\displaystyle \theta _{2}(q)=2q^{\frac {1}{4}}\sum _{n=0}^{\infty }q^{n(n+1)}\quad ,\quad \theta _{3}(q)=\sum _{n\in {\mathbb {Z} }}q^{n^{2}}}$

The representation is of the type

${\displaystyle {\mathcal {F}}_{\Delta }^{(s)}(\{\Delta _{i}\}|z)=(16q)^{\Delta -{\frac {1}{4}}Q^{2}}z^{{\frac {1}{4}}Q^{2}-\Delta _{1}-\Delta _{2}}(1-z)^{{\frac {1}{4}}Q^{2}-\Delta _{1}-\Delta _{4}}\theta _{3}(q)^{3Q^{2}-4(\Delta _{1}+\Delta _{2}+\Delta _{3}+\Delta _{4})}H_{\Delta }(\{\Delta _{i}\}|q)\ .}$

The function ${\displaystyle H_{\Delta }(\{\Delta _{i}\}|q)}$ is a power series in ${\displaystyle q}$, which is recursively defined by

${\displaystyle H_{\Delta }(\{\Delta _{i}\}|q)=1+\sum _{m,n=1}^{\infty }{\frac {(16q)^{mn}}{\Delta -\Delta _{(m,n)}}}R_{m,n}H_{\Delta _{(m,-n)}}(\{\Delta _{i}\}|q)\ .}$

In this formula, the positions ${\displaystyle \Delta _{(m,n)}}$ of the poles are the dimensions of degenerate representations, which correspond to the momentums

${\displaystyle P_{(m,n)}={\frac {1}{2}}\left(mb+nb^{-1}\right)\ .}$

The residues ${\displaystyle R_{m,n}}$ are given by

${\displaystyle R_{m,n}={\frac {2P_{(0,0)}P_{(m,n)}}{\prod _{r=1-m}^{m}\prod _{s=1-n}^{n}2P_{(r,s)}}}\prod _{r{\overset {2}{=}}1-m}^{m-1}\prod _{s{\overset {2}{=}}1-n}^{n-1}\prod _{\pm }(P_{2}\pm P_{1}+P_{(r,s)})(P_{3}\pm P_{4}+P_{(r,s)})\ ,}$

where the superscript in ${\displaystyle {\overset {2}{=}}}$ indicates a product that runs by increments of ${\displaystyle 2}$. The recursion relation for ${\displaystyle H_{\Delta }(\{\Delta _{i}\}|q)}$ can be solved, giving rise to an explicit (but impractical) formula.^[2][9]

While the coefficients of the power series ${\displaystyle H_{\Delta }(\{\Delta _{i}\}|q)}$ need not be positive in unitary theories, the coefficients of ${\displaystyle \prod _{k=1}^{\infty }(1-q^{2k})^{-{\frac {1}{2}}}H_{\Delta }(\{\Delta _{i}\}|q)}$ are positive, due to this combination's interpretation in terms of sums of states in the pillow geometry.^[10]

The recursive representation can be seen as an expansion around ${\displaystyle \Delta =\infty }$. It is sometimes called the ${\displaystyle \Delta }$-recursion, in order to distinguish it from the ${\displaystyle c}$-recursion: another recursive representation, also due to Alexei Zamolodchikov, which expands around ${\displaystyle c=\infty }$. Both representations can be generalized to ${\displaystyle N}$-point Virasoro conformal blocks on arbitrary Riemann surfaces.^[11]

From the relation to instanton counting

The Alday–Gaiotto–Tachikawa relation between two-dimensional conformal field theory and supersymmetric gauge theory, more specifically, between the conformal blocks of Liouville theory and Nekrasov partition functions^[12] of supersymmetric gauge theories in four dimensions, leads to combinatorial expressions for conformal blocks as sums over Young diagrams. Each diagram can be interpreted as a state in a representation of the Virasoro algebra, times an abelian affine Lie algebra.^[13]

Zero-point blocks on the torus

A zero-point block does not depend on field positions, but it depends on the moduli of the underlying Riemann surface. In the case of the torus

${\displaystyle {\frac {\mathbb {C} }{\mathbb {Z} +\tau \mathbb {Z} }},}$

that dependence is better written through ${\displaystyle q=e^{2\pi i\tau }}$ and the zero-point block associated to a representation ${\displaystyle {\mathcal {R}}}$ of the Virasoro algebra is

${\displaystyle \chi _{\mathcal {R}}(\tau )=\operatorname {Tr} _{\mathcal {R}}q^{L_{0}-{\frac {c}{24}}},}$

where ${\displaystyle L_{0}}$ is a generator of the Virasoro algebra. This coincides with the character of ${\displaystyle {\mathcal {R}}.}$ The characters of some highest-weight representations are:^[1]

• Verma module with conformal dimension ${\displaystyle \Delta ={\tfrac {c-1}{24}}-P^{2}}$:

${\displaystyle \chi _{P}(\tau )={\frac {q^{-P^{2}}}{\eta (\tau )}},}$

where ${\displaystyle \eta (\tau )}$ is the Dedekind eta function.

• Degenerate representation with the momentum ${\displaystyle P_{(r,s)}}$:

${\displaystyle \chi _{(r,s)}(\tau )=\chi _{P_{(r,s)}}(\tau )-\chi _{P_{(r,-s)}}(\tau ).}$

• Fully degenerate representation at rational ${\displaystyle b^{2}=-{\tfrac {p}{q}}}$:

${\displaystyle \chi _{(r,s)}(\tau )=\sum _{k\in \mathbb {Z} }\left(\chi _{P_{(r,s)}+ik{\sqrt {pq}}}(\tau )-\chi _{P_{(r,-s)}+ik{\sqrt {pq}}}(\tau )\right).}$

The characters transform linearly under the modular transformations:

${\displaystyle \tau \to {\frac {a\tau +b}{c\tau +d}},\qquad {\begin{pmatrix}a&b\\c&d\end{pmatrix}}\in SL_{2}(\mathbb {Z} ).}$

In particular their transformation under ${\displaystyle \tau \to -{\tfrac {1}{\tau }}}$ is described by the modular S-matrix. Using the S-matrix, constraints on a CFT's spectrum can be derived from the modular invariance of the torus partition function, leading in particular to the ADE classification of minimal models.^[14]

One-point blocks on the torus

An arbitrary one-point block on the torus can be written in terms of a four-point block on the sphere at a different central charge. This relation maps the modulus of the torus to the cross-ratio of the four points' positions, and three of the four fields on the sphere have the fixed momentum ${\displaystyle P_{(0,{\frac {1}{2}})}={\tfrac {1}{4b}}}$:^[15][16]

${\displaystyle H_{P'}^{\text{torus}}(P_{1}|q^{2})=H_{P}\left(\left.{\tfrac {1}{4b}},P_{2},{\tfrac {1}{4b}},{\tfrac {1}{4b}}\right|q\right)\quad {\text{with}}\quad \left\{{\begin{array}{l}b={\frac {b'}{\sqrt {2}}}\\P_{2}={\frac {P_{1}}{\sqrt {2}}}\\P={\sqrt {2}}P'\end{array}}\right.}$

where

• ${\displaystyle H_{P_{s}}\left(\left.P_{1},P_{2},P_{3},P_{4}\right|q\right)}$ is the non-trivial factor of the sphere four-point block in Zamolodchikov's recursive representation, written in terms of momentums ${\displaystyle P_{i}}$ instead of dimensions ${\displaystyle \Delta _{i}}$.
• ${\displaystyle H_{P}^{\text{torus}}(P_{1}|q)}$ is the non-trivial factor of the torus one-point block ${\displaystyle {\mathcal {F}}_{\Delta }^{\text{torus}}(\Delta _{1}|q)=q^{\Delta -{\frac {c-1}{24}}}\eta (q)^{-1}H_{\Delta }^{\text{torus}}(\Delta _{1}|q)}$, where ${\displaystyle \eta (q)}$ is the Dedekind eta function, the modular parameter ${\displaystyle \tau }$ of the torus is such that ${\displaystyle q=e^{2\pi i\tau }}$, and the field on the torus has the dimension ${\displaystyle \Delta _{1}}$.

The recursive representation of one-point blocks on the torus is^[17]

${\displaystyle H_{\Delta }^{\text{torus}}(\Delta _{1}|q)=1+\sum _{m,n=1}^{\infty }{\frac {q^{mn}}{\Delta -\Delta _{(m,n)}}}R_{m,n}^{\text{torus}}H_{\Delta _{(m,-n)}}^{\text{torus}}(\Delta _{1}|q)\ ,}$

where the residues are

${\displaystyle R_{m,n}^{\text{torus}}={\frac {2P_{(0,0)}P_{(m,n)}}{\prod _{r=1-m}^{m}\prod _{s=1-n}^{n}2P_{(r,s)}}}\prod _{r{\overset {2}{=}}1-2m}^{2m-1}\prod _{s{\overset {2}{=}}1-2n}^{2n-1}\left(P_{1}+P_{(r,s)}\right)\ .}$

Under modular transformations, one-point blocks on the torus behave as

${\displaystyle {\mathcal {F}}_{P}^{\text{torus}}\left(P_{1}|-{\tfrac {1}{\tau }}\right)=\int _{i\mathbb {R} }dP'\ S_{P,P'}(P_{1}){\mathcal {F}}_{P'}^{\text{torus}}\left(P_{1}|\tau \right)\ ,}$

where the modular kernel is^[18][19]

${\displaystyle S_{P,P'}(P_{1})={\frac {\sqrt {2}}{S_{b}({\frac {Q}{2}}+P_{1})}}\prod _{\pm }{\frac {\Gamma _{b}(Q\pm 2P)}{\Gamma _{b}(\pm 2P')}}{\frac {\Gamma _{b}({\frac {Q}{2}}-P_{1}\pm 2P')}{\Gamma _{b}({\frac {Q}{2}}-P_{1}\pm 2P)}}\int _{i\mathbb {R} }{\frac {du}{i}}\ e^{4\pi iPu}\prod _{\pm ,\pm }S_{b}\left({\tfrac {Q}{4}}+{\tfrac {P_{1}}{2}}\pm u\pm P'\right)\ .}$

Hypergeometric blocks

For a four-point function on the sphere

${\displaystyle \left\langle V_{\langle 2,1\rangle }(x)\prod _{i=1}^{3}V_{\Delta _{i}}(z_{i})\right\rangle }$

where one field has a null vector at level two, the second-order BPZ equation reduces to the hypergeometric equation. A basis of solutions is made of the two s-channel conformal blocks that are allowed by the fusion rules, and these blocks can be written in terms of the hypergeometric function,

${\displaystyle {\begin{aligned}{\mathcal {F}}_{P_{1}+\epsilon {\frac {b}{2}}}^{(s)}(z)&=z^{{\frac {1}{2}}+{\frac {b^{2}}{2}}+b\epsilon P_{1}}(1-z)^{{\frac {1}{2}}+{\frac {b^{2}}{2}}+bP_{3}}\\&\times F\left({\tfrac {1}{2}}+b(\epsilon P_{1}+P_{2}+P_{3}),{\tfrac {1}{2}}+b(\epsilon P_{1}-P_{2}+P_{3}),1+2b\epsilon P_{1},z\right),\end{aligned}}}$

with ${\displaystyle \epsilon \in \{+,-\}.}$ Another basis is made of the two t-channel conformal blocks,

${\displaystyle {\begin{aligned}{\mathcal {F}}_{P_{3}+\epsilon {\frac {b}{2}}}^{(t)}(z)&=z^{{\frac {1}{2}}+{\frac {b^{2}}{2}}+bP_{1}}(1-z)^{{\frac {1}{2}}+{\frac {b^{2}}{2}}+b\epsilon P_{3}}\\&\times F\left({\tfrac {1}{2}}+b(P_{1}+P_{2}+\epsilon P_{3}),{\tfrac {1}{2}}+b(P_{1}-P_{2}+\epsilon P_{3}),1+2b\epsilon P_{3},1-z\right).\end{aligned}}}$

The fusing matrix is the matrix of size two such that

${\displaystyle {\mathcal {F}}_{P_{1}+\epsilon _{1}{\frac {b}{2}}}^{(s)}(x)=\sum _{\epsilon _{3}=\pm }F_{\epsilon _{1},\epsilon _{3}}{\mathcal {F}}_{P_{3}+\epsilon _{3}{\frac {b}{2}}}^{(t)}(x),}$

whose explicit expression is

${\displaystyle F_{\epsilon _{1},\epsilon _{3}}={\frac {\Gamma (1-2b\epsilon _{1}P_{1})\Gamma (2b\epsilon _{3}P_{3})}{\prod _{\pm }\Gamma ({\frac {1}{2}}+b(-\epsilon _{1}P_{1}\pm P_{2}+\epsilon _{3}P_{3}))}}.}$

Hypergeometric conformal blocks play an important role in the analytic bootstrap approach to two-dimensional CFT.^[20][21]

Solutions of the Painlevé VI equation

If ${\displaystyle c=1,}$ then certain linear combinations of s-channel conformal blocks are solutions of the Painlevé VI nonlinear differential equation.^[22] The relevant linear combinations involve sums over sets of momentums of the type ${\displaystyle P_{s}+i\mathbb {Z} .}$ This allows conformal blocks to be deduced from solutions of the Painlevé VI equation and vice versa. This also leads to a relatively simple formula for the fusing matrix at ${\displaystyle c=1.}$^[23] Curiously, the ${\displaystyle c=\infty }$ limit of conformal blocks is also related to the Painlevé VI equation.^[24] The relation between the ${\displaystyle c=\infty }$ and the ${\displaystyle c=1}$ limits, mysterious on the conformal field theory side, is explained naturally in the context of four dimensional gauge theories, using blowup equations,^[25][26] and can be generalized to more general pairs ${\displaystyle c,c'}$of central charges.

Other representations of the Virasoro algebra

The Virasoro conformal blocks that are described in this article are associated to a certain type of representations of the Virasoro algebra: highest-weight representations, in other words Verma modules and their cosets.^[2] Correlation functions that involve other types of representations give rise to other types of conformal blocks. For example:

• Logarithmic conformal field theory involves representations where the Virasoro generator ${\displaystyle L_{0}}$ is not diagonalizable, which give rise to blocks that depend logarithmically on field positions.
• Representations can be built from states on which some annihilation modes of the Virasoro algebra act diagonally, rather than vanishing. The corresponding conformal blocks have been called irregular conformal blocks.^[27]

Larger symmetry algebras

In a theory whose symmetry algebra is larger than the Virasoro algebra, for example a WZW model or a theory with W-symmetry, correlation functions can in principle be decomposed into Virasoro conformal blocks, but that decomposition typically involves too many terms to be useful. Instead, it is possible to use conformal blocks based on the larger algebra: for example, in a WZW model, conformal blocks based on the corresponding affine Lie algebra, which obey Knizhnik–Zamolodchikov equations.