Superintegrability for ($\beta$-deformed) partition function hierarchies with $W$-representations

We construct the ($\beta$-deformed) partition function hierarchies with $W$-representations. Based on the $W$-representations, we analyze the superintegrability property and derive their character expansions with respect to the Schur functions and Jack polynomials, respectively. Some well known superintegrable matrix models such as the Gaussian hermitian one-matrix model (in the external field), $N\times N$ complex matrix model, $\beta$-deformed Gaussian hermitian and rectangular complex matrix models are contained in the constructed hierarchies.


Introduction
Recently there has been increasing interest in the superintegrability for matrix models [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The superintegrability means that for the character expansions of the matrix models, the average of a properly chosen symmetric function is proportional to ratios of symmetric functions on a proper locus, i.e., < character >∼ character. A wide range of matrix models are known to be superintegrable, such as the (deformed) Gaussian hermitian and complex matrix models [1][2][3][4], (Hurwitz-)Kontsevich matrix models [5,6], unitary matrix models [7], fermionic matrix models [8,9], and even some non-Gaussian matrix models [10,11]. The constraints for matrix models are useful to analyze the structures of matrix models. For the Gaussian hermitian one-matrix model, its character expansion with respect to the Schur functions can be derived recursively from a single w-constraint [13]. There are the Virasoro constraints (with higher algebraic structures) for matrix models. They can be applied to analyze the character expansions of the matrix models as well, such as the Gaussian hermitian one-matrix, complex matrix and fermionic matrix models [8,14].
W -representations of matrix models give the dual expressions for the partition functions through differentiation rather than integration [5]. More precisely, the partition functions are realized by acting on elementary functions with exponents of the given W -operators. For the Gaussian tensor model [19] and (fermionic) rainbow tensor models [8,20], they can still be expressed as the W -representations. Recently it was shown that the superintegrability for (βdeformed) matrix models can be analyzed from their W -representations [15,16]. In this paper, we will construct the partition function hierarchies with W -representations and analyze the superintegrability property.

Partition function hierarchies with W -representations
The Hurwitz-Kontsevich matrix model is a deformation of the Kontsevich model, which can be used to describe the Hurwitz numbers and Hodge integrals over the moduli space of complex curves [5,21,22]. It is the special case of the more general Hurwitz partition functions [23][24][25]. The Hurwitz-Kontsevich model is generated by the exponent of the Hurwitz operator W 0 acting on the function e p 1 /e tN , where t is a deformation parameter, the Hurwitz operator W 0 is given by and c λ = (i,j)∈λ (N − i + j).
The partition function (1) possesses the matrix model representation [5] where ψ is an N × N matrix and the time variables p k = Trψ k . Let us define the operator Using the actions and we have where λ + are the Young diagrams obtained by adding one square = (i , j ) to λ. Let us set The action of W −1 on the Schur functions is In terms of W −1 and E 1 , we introduce a series of operators The actions of W −n with n ≥ 1 on the Schur functions are given by where we denote α = 1 + δ n,1 in this paper for later convenience, λ + 1 + · · · + n are the Young diagrams obtained by adding n squares to λ, A λ+ 1 +···+ n λ are the coefficients in the actions It is known that , with the Littlewood-Richardson coefficients a λ µ,ν defined by S µ S ν = λ a λ µ,ν S λ [26]. The actions (11) can be proved inductively by using the relations p n = 1 n−1 [E 1 , p n−1 ], n ≥ 2. Let us introduce the partition function hierarchy with W -representations It is straightforward to calculate the powers of W −n acting on S λ with λ = ∅, leading to the explicit results where we have used the hook length formula We see that Z −1 {p} and Z −2 {p} are the N × N complex matrix model [1,27] and Gaussian hermitian one-matrix model [1,5], respectively, Note that for any partition function of the form [25] with the arbitrary function f and parametersp k , it is a τ -function of the KP hierarchy. It is clear that the partition function hierarchy (15) gives the τ -functions of the KP hierarchy.
Similarly, we define the operator and where λ − are the Young diagrams obtained by removing one square = (i , j ) from λ. We set There is the action In terms of W 1 and E −1 , we introduce a series of operators The actions of W n on the Schur functions are given by where λ − 1 − · · · − n are the Young diagrams obtained by removing n squares from λ, A λ− 1 −···− n λ = n k=1 (−1) k a λ λ− 1 −···− n,(k,1 n−k ) are the coefficients in the actions The actions (23) can be proved inductively by using the relations ∂ ∂pn = 1 n [E −1 , ∂ ∂p n−1 ], n ≥ 2. It is interesting to note that there are the same actions as (23) for the operators W − n given by [18] where H is an N × N matrix. Taking p k = TrH k , we can rewrite the operators (18), (20) and W 2 in (22) as where H T is the transpose of the matrix H.
Since (22) Since there are the actions where S λ/µ are the skew Schur functions, we obtain the character expansions for the partition function hierarchy (27) Here we have used the Cauchy formula e ∞ k=1 1 k p kpk = λ S λ {p}S λ {p}. When particularized to the n = 2 case in (29), it gives the character expansion [15] of Gaussian hermitian one-matrix model in the external field [5] where p k = TrM k 2 .
For the operator it gives the W -operator in the W -representations of β-deformed Gaussian hermitian matrix model [3]. By the actions (38) and (42), we have where B λ+ 1 + 2 λ are the coefficients in (40). Let us introduce a series of operators There are the actions where B λ+ 1 +···+ n λ are the coefficients in The actions (46) can be proved inductively by using the relations p n = 1 n−1 [Ē 1 , p n−1 ], n ≥ 2. Let us introduce the partition function hierarchy with W -representations It is straightforward to calculate the power of W −n acting on J λ with λ = ∅, leading to the explicit result Using the hook length formula we further obtain Let us turn to construct the operator where the operatorĒ −1 is given bȳ We have the action where B λ− λ are the coefficients in Let us introduce the operators We have the actions where B λ− 1 −···− n λ are the coefficients in We introduce the partition function hierarchy with W -representations

Summary
It was known that the Hurwitz-Kontsevich matrix model (1) can be expressed as the exponent of the Hurwitz operator W 0 acting on the function e p 1 /e tN . In terms of the Hurwitz operator W 0 , p 1 and ∂ ∂p 1 , we have constructed the partition function hierarchies with W -representations. Based on the W -representations, we showed that these partition functions can be expressed as the character expansions with respect to the Schur functions. It was noted that the character expansions of hierarchy (15) give the τ -functions of the KP hierarchy, and the N × N complex matrix and Gaussian hermitian one-matrix models are contained in the hierarchy (15). For the constructed partition function hierarchy (29), it contains the Gaussian hermitian onematrix model in the external field. We have also extended the Hurwitz-Kontsevich matrix model (1) to the β-deformed case. Similarly, the β-deformed partition function hierarchies with W -representations were constructed and their character expansions with respect to the Jack polynomials were presented as well. The β-deformed rectangular complex and Gaussian hermitian matrix models are contained in the hierarchy (52). Searching for the matrix model representations of the partition functions in the hierarchies presented in this paper would merit further investigations. Furthermore, it would be interesting to construct q, t-deformed partition function hierarchies with W -representations and study their character expansions with respect to the Macdonald polynomials.