Abstract: 
This article formalises a proof of the maximumflow minimalcut
theorem for networks with countably many edges. A network is a
directed graph with nonnegative realvalued edge labels and two
dedicated vertices, the source and the sink. A flow in a network
assigns nonnegative real numbers to the edges such that for all
vertices except for the source and the sink, the sum of values on
incoming edges equals the sum of values on outgoing edges. A cut is a
subset of the vertices which contains the source, but not the sink.
Our theorem states that in every network, there is a flow and a cut
such that the flow saturates all the edges going out of the cut and is
zero on all the incoming edges. The proof is based on the paper
The MaxFlow MinCut theorem for countable networks by
Aharoni et al. Additionally, we prove a characterisation of the
lifting operation for relations on discrete probability distributions,
which leads to a concise proof of its distributivity over relation
composition. 
BibTeX: 
@article{MFMC_CountableAFP,
author = {Andreas Lochbihler},
title = {A Formal Proof of the MaxFlow MinCut Theorem for Countable Networks},
journal = {Archive of Formal Proofs},
month = may,
year = 2016,
note = {\url{http://isaafp.org/entries/MFMC_Countable.html},
Formal proof development},
ISSN = {2150914x},
}
