 The problem with this result is that it is valid only for sum functions, and in particular only if the energy function of the system is itself a sum function, which is not the case when particles interact. So the question is how this result can be generalised to more realistic cases. This problem stands at the starting point of a research programme now known as the thermodynamic limit, championed, among others, by Lanford, Mazur, Ruelle, and van der Linden (see van Lith (2001) for a survey). Its leading question is whether one can still prove ‘Khinchin-like’ results in the case of energy function with interaction  terms.[19]  Results of this kind can be proven in the limit for \(n \rightarrow \infty\), if also the volume \(V\) of the system tends towards infinity in such a way that the number-density \(n/V\) remains constant.