# Weingarten function

In mathematics, Weingarten functions are rational functions indexed by partitions of integers that can be used to calculate integrals of products of matrix coefficients over classical groups. They were first studied by Weingarten (1978) who found their asymptotic behavior, and named by Collins (2003), who evaluated them explicitly for the unitary group.

## Unitary groups

Weingarten functions are used for evaluating integrals over the unitary group U$_d$ of products of matrix coefficients of the form
    $\int _{U_{d}}U_{i_{1}j_{1}}\cdots U_{i_{q}j_{q}}U_{i_{1}^{\prime }j_{1}^{\prime }}^{*}\cdots U_{i_{q}^{\prime }j_{q}^{\prime }}^{*}dU,$
where $*$ denotes complex conjugation. Note that $U_{ji}^{*}=(U^{\dagger })_{ij}$ where $U^{\dagger }$ is the conjugate transpose of $U$, so one can interpret the above expression as being for the $i_{1}j_{1}\ldots i_{q}j_{q}j'_{1}i'_{1}\ldots j'_{q}i'_{q}$ matrix element of $U\otimes \cdots \otimes U\otimes U^{\dagger }\otimes \cdots \otimes U^{\dagger }$.
This integral is equal to
    $\sum _{\sigma ,\tau \in S_{q}}\delta _{i_{1}i_{\sigma (1)}^{\prime }}\cdots \delta _{i_{q}i_{\sigma (q)}^{\prime }}\delta _{j_{1}j_{\tau (1)}^{\prime }}\cdots \delta _{j_{q}j_{\tau (q)}^{\prime }}W\!g(\sigma \tau ^{-1},d)$
where Wg is the Weingarten function, given by
    $W\!g(\sigma ,d)={\frac {1}{q!^{2}}}\sum _{\lambda }{\frac {\chi ^{\lambda }(1)^{2}\chi ^{\lambda }(\sigma )}{s_{\lambda ,d}(1)}}$
where the sum is over all partitions λ of q (Collins 2003). Here χ$^λ$ is the character of S$_q$ corresponding to the partition λ and s is the Schur polynomial of λ, so that s$_{λd}$(1) is the dimension of the representation of U$_d$ corresponding to λ.
The Weingarten functions are rational functions in d. They can have poles for small values of d, which cancel out in the formula above. There is an alternative inequivalent definition of Weingarten functions, where one only sums over partitions with at most d parts. This is no longer a rational function of d, but is finite for all positive integers d. The two sorts of Weingarten functions coincide for d larger than q, and either can be used in the formula for the integral.

### Values of the Weingarten function for simple permutations

The first few Weingarten functions Wg(σ, d) are
    $\displaystyle W\!g(,d)=1$ (The trivial case where q = 0)
    $\displaystyle W\!g(1,d)={\frac {1}{d}}$
    $\displaystyle W\!g(2,d)={\frac {-1}{d(d^{2}-1)}}$
    $\displaystyle W\!g(1^{2},d)={\frac {1}{d^{2}-1}}$
    $\displaystyle W\!g(3,d)={\frac {2}{d(d^{2}-1)(d^{2}-4)}}$
    $\displaystyle W\!g(21,d)={\frac {-1}{(d^{2}-1)(d^{2}-4)}}$
    $\displaystyle W\!g(1^{3},d)={\frac {d^{2}-2}{d(d^{2}-1)(d^{2}-4)}}$
    $\displaystyle W\!g(4,d)={\frac {-5}{d(d^{2}-1)(d^{2}-4)(d^{2}-9)}}$
    $\displaystyle W\!g(31,d)={\frac {2d^{2}-3}{d^{2}(d^{2}-1)(d^{2}-4)(d^{2}-9)}}$
    $\displaystyle W\!g(2^{2},d)={\frac {d^{2}+6}{d^{2}(d^{2}-1)(d^{2}-4)(d^{2}-9)}}$
    $\displaystyle W\!g(21^{2},d)={\frac {-1}{d(d^{2}-1)(d^{2}-9)}}$
    $\displaystyle W\!g(1^{4},d)={\frac {d^{4}-8d^{2}+6}{d^{2}(d^{2}-1)(d^{2}-4)(d^{2}-9)}}$
where permutations σ are denoted by their cycle shapes.
There exist computer algebra programs to produce these expressions.

### Explicit expressions for the integrals in the first cases

The explicit expressions for the integrals of first- and second-degree polynomials, obtained via the formula above, are:$\int _{U_{d}}dUU_{ij}{\bar {U}}_{k\ell }=\delta _{ik}\delta _{j\ell }\operatorname {Wg} (1,d)={\frac {\delta _{ik}\delta _{j\ell }}{d}}.$$\int _{U_{d}}dUU_{ij}U_{k\ell }{\bar {U}}_{mn}{\bar {U}}_{pq}=(\delta _{im}\delta _{jn}\delta _{kp}\delta _{\ell q}+\delta _{ip}\delta _{jq}\delta _{km}\delta _{\ell n})\operatorname {Wg} (1^{2},d)+(\delta _{im}\delta _{jq}\delta _{kp}\delta _{\ell n}+\delta _{ip}\delta _{jn}\delta _{km}\delta _{\ell q})\operatorname {Wg} (2,d).$

### Asymptotic behavior

For large d, the Weingarten function Wg has the asymptotic behavior
    $W\!g(\sigma ,d)=d^{-n-|\sigma |}\prod _{i}(-1)^{|C_{i}|-1}c_{|C_{i}|-1}+O(d^{-n-|\sigma |-2})$
where the permutation σ is a product of cycles of lengths C$_i$, and c$_n$ = (2n)!/n!(n + 1)! is a Catalan number, and |σ| is the smallest number of transpositions that σ is a product of. There exists a diagrammatic method to systematically calculate the integrals over the unitary group as a power series in 1/d.

## Orthogonal and symplectic groups

For orthogonal and symplectic groups the Weingarten functions were evaluated by Collins & Śniady (2006). Their theory is similar to the case of the unitary group. They are parameterized by partitions such that all parts have even size.