# Fixed-parameter tractability of counting small minimum $(S,T)$-cuts

## TL;DR

This paper introduces a fixed-parameter tractable algorithm for counting minimum edge (S,T)-cuts in undirected graphs, using a novel graph transformation called drainage to efficiently enumerate all such cuts.

## Contribution

It presents the first fixed-parameter tractable algorithm for counting minimum (S,T)-cuts parameterized by their size, utilizing a new graph transformation and combinatorial insights.

## Key findings

- Algorithm operates in 2^{O(p^2)}n^{O(1)} time
- Transforms graph to reveal successive minimum (S,T)-cuts
- Can be adapted for FPT sampling of minimum (S,T)-cuts

## Abstract

The parameterized complexity of counting minimum cuts stands as a natural question because Ball and Provan showed its #P-completeness. For any undirected graph $G=(V,E)$ and two disjoint sets of its vertices $S,T$, we design a fixed-parameter tractable algorithm which counts minimum edge $(S,T)$-cuts parameterized by their size $p$. Our algorithm operates on a transformed graph instance. This transformation, called drainage, reveals a collection of at most $n=\left| V \right|$ successive minimum $(S,T)$-cuts $Z_i$. We prove that any minimum $(S,T)$-cut $X$ contains edges of at least one cut $Z_i$. This observation, together with Menger's theorem, allows us to build the algorithm counting all minimum $(S,T)$-cuts with running time $2^{O(p^2)}n^{O(1)}$. Initially dedicated to counting minimum cuts, it can be modified to obtain an FPT sampling of minimum edge $(S,T)$-cuts.