From Wikipedia: "Number theory (or arithmetic or higher arithmetic in older usage) is a branch of pure mathematics devoted primarily to the study of the integers."

First see examples in the [MO-Q][1] "Where is number theory used in the rest of mathematics?" 

I commented there, "Goldman in his book The Queen of Mathematics alludes to an article by Weinberg where the partition function of number theory is related to the states of a vibrating string."

With a more combinatorial flavor:

Solutions to the inviscid Burgers' the KdV, and the KP equations of hydrodynamics and to the general evolution equations for flow fields generated by tangent vectors are related to classic integer arrays. (Not so surprising since the iterated operators $(x^{m+1}d/dx)^n$ are related to classic integer arrays and combinatorics.) 

The integers relate to solutions of the Burgers' equation through the combinatorics of the associahedra and its relations to compositional inversion through OEIS [A133437][2] as sketched in the answer to [MO-Q][3] "Why is there a connection between enumerative geometry and nonlinear waves?" 

A bivariate e.g.f. for the Eulerian numbers ([A008292][4]/[A123125][5]) with its associated quadratic ($sl_2$) infinigen provides a soliton solution of the 1-D KdV equation. (The Eulerians are rife with ($A_n$ and $B_n$) connnections to enumerative algebraic geometry, as discussed by Hirzebruch, Losev and Manin, Batryev and Blume, Cohen, et al.)


Lauren Williams in "Enumeration of totally positive Grassmann cells" develops a polynomial generating function $A_{k,n}(q)$ whose $q^d$ coefficient is the number [A046802][6] of totally positive cells in $G^+(k,n)$ that have dimension $d$ and goes on to show that for the binomial transform $\hat{E}_{k,n}(q)=q^{k-n}\sum^n_{i=0} (-1)^i \binom{n}{i} A_{k,n-i}(q)$ that $\hat{E}_{k,n(}(1)=E_{k,n}$, the Eulerian numbers [A008292][4], and $\hat{E}_{k,n}(0)=N_{k,n}$, the Narayana numbers [A001263][7]. She reiterates this in her presentation "The Positive Grassmannian (a mathematician's perspective)" and relates $G^+$ to soliton shallow-water-wave solutions of a KP equation, noting also the roles of $G^+$ in computing scattering amplitudes in string theory, a relation to free probability, and the occurrence of the Eulerians and Narayanaians in the BCFW recurrence and twistor string theory. (See links to Williams' papers in [A046802][6].)

The refined Eulerian numbers [A145271][8] arise in the series expansion for flow fields generated by [exponentiation of tangent vectors][9].

The conservation equations associated to the Burgers' and KdV equations also have connections to classic integer sequences. See also "[Set partitions and integrable hierarchies][10]" by V.E. Adler. 

See also some refs and comments on the relation of the cycle index polynomials for the symmetric group $S_n$ [A036039][11] (refined Stirling polynomials of the first kind, related to the elementary Schur polynomials and Faber polynomials) to tau functions and integrable hierarchies (and zeta functions). The Faber polynomials [A263916][12] are also related to integrable systems, evolution equations, and number theoretic relations.


  [1]: https://mathoverflow.net/questions/90700/where-is-number-theory-used-in-the-rest-of-mathematics
  [2]: https://oeis.org/A133437
  [3]: https://mathoverflow.net/questions/145555/why-is-there-a-connection-between-enumerative-geometry-and-nonlinear-waves/181534#181534
  [4]: https://oeis.org/A008292
  [5]: https://oeis.org/A123125
  [6]: https://oeis.org/A046802
  [7]: https://oeis.org/A001263
  [8]: https://oeis.org/A145271
  [9]: https://mathoverflow.net/questions/339061/guises-of-the-refined-eulerian-numbers-generated-by-tangent-vectors-oeis-a1452
  [10]: https://arxiv.org/abs/1510.02900
  [11]: https://oeis.org/A036039
  [12]: https://oeis.org/A263916