A. Church, in his classical paper An unsolvable problem in elementary number theory in American Journal of Mathematics Vol. 58 No. 2. (1936), pp. 345-363, (available here), wrote:
There is a class of problems of elementary number theory which can be stated in the form that it is required to find an effectively calculable function $f$ of $n$ positive integers, such that $f(x_1,x_2,\dots,x_n)=2$ is a necessary and sufficient condition for the truth of a certain proposition of elementary number theory involving $x_1,x_2,\dots,x_n$ as free variables.
As an example he gave:
[...] [A] problem of this class is, for instance, the problem of topology, to find a complete set of effectively calculable invariants of closed three-dimensional simplicial manifolds under homeomorphisms. This problem can be interpreted as a problem of elementary number theory in view of the fact that topological complexes are representable by matrices of incidence. In fact, as is well known, the property of a set of incidence matrices that it represent a closed three-dimensional manifold, and the property of two sets of incidence matrices that they represent homeomorphic complexes, can both be described in purely number-theoretic terms.
I'd like to understand the gist of this example, although it's not needed in the rest of the paper. What's a "topological complex"? Is that a simplicial complex? In which sense can it be represented as incidence matrices, assuming it's the same concept from graph theory?