The tacnode Riemann-Hilbert problem is a 4 × 4 matrix valued RH problem that appears in the description of the local behavior of two touching groups of non-intersecting Brownian motions. The same RH problem was also found by Duits and Geudens to describe a new critical regime in the two-matrix model.Delvaux gave integral representations for some of the entries of the 4 × 4 matrix. We complement this work by presenting integral representations for all of the entries. As a consequence we give an explicit formula for the Duits-Geudens critical kernel * Recently, Delvaux [10] made the connection between the two sets of formulas by presenting integral representations for some of the entries of the solution of the tacnode RH problem. These entries are exactly the ones that play a role for the tacnode kernel in [12]. With these explicit formulas Delvaux could make the connection between the formulas in [12] and the ones by Ferrari and Vető [16] for the asymmetric tacnode. The paper [10] was inspired by the paper [6] by Baik, Liechty, and Schehr, where a connection between different sets of formulas for the maximal height and position of the Airy 2 process was made.The aim of this paper is to complement the work of [10] by providing integral representations for all the entries of the tacnode RH problem. Some of these remaining entries appear in the description of a critical kernel appearing in the two-matrix model as shown by Duits and Geudens [13]. We therefore find explicit integral formulas for the Duits-Geudens critical kernel.In section 2 we recall the tacnode RH problem with some of its properties, and in particular the connection with the Hastings-McLeod solution of Painlevé II. The main results of this paper are stated in Theorems 2.5 and 2.8 below. We compare the solution of the tacnode RH problem with the explicit solution of the usual 2 × 2 matrix-valued RH problem for the Hastings-McLeod solution in section 2.6.The proofs of the results are in section 3. A key role is played by Lemma 3.2 that describes solutions to a certain ODE system (3.2). The proof of this lemma follows along the lines of certain proofs in [10]. We give full details about the calculations in section 5. Following [10] we briefly mention the tacnode kernel in section 4.1. The implications of Theorem 2.8 for the Duits-Geudens critical kernel are discussed in section 4.A variation of the tacnode RH problem for the hard-edge tacnode and the chiral two-matrix model appears in [9,11]. It may be possible that explicit integral representations for the solution of these RH problems can be found as well. Other recent contributions [1,3,7,19] discuss further connections and properties of the tacnode process.
Statement of results
The tacnode RH problemThe tacnode RH problem asks for a 4 × 4 matrix-valued function M : C \ Γ M → C 4×4 Proposition 2.2 was proved in the case τ = 0 by Delvaux, Kuijlaars, and Zhang [12], and in the case r 1 = r 2 = 1, s 1 = s 2 with general τ by Duits and Geudens [13]. The proof in [13] extends to the general case as noted...