Skip to contents

Listing all minimum \((s,t)\)-cuts of a directed graph, for given \(s\) and \(t\).

Usage

st_min_cuts(graph, source, target, capacity = NULL)

Arguments

graph

The input graph. It must be directed.

source

The id of the source vertex.

target

The id of the target vertex.

capacity

Numeric vector giving the edge capacities. If this is NULL and the graph has a weight edge attribute, then this attribute defines the edge capacities. For forcing unit edge capacities, even for graphs that have a weight edge attribute, supply NA here.

Value

A list with entries:

value

Numeric scalar, the size of the minimum cut(s).

cuts

A list of numeric vectors containing edge ids. Each vector is a minimum \((s,t)\)-cut.

partition1s

A list of numeric vectors containing vertex ids, they correspond to the edge cuts. Each vertex set is a generator of the corresponding cut, i.e. in the graph \(G=(V,E)\), the vertex set \(X\) and its complementer \(V-X\), generates the cut that contains exactly the edges that go from \(X\) to \(V-X\).

Details

Given a \(G\) directed graph and two, different and non-ajacent vertices, \(s\) and \(t\), an \((s,t)\)-cut is a set of edges, such that after removing these edges from \(G\) there is no directed path from \(s\) to \(t\).

The size of an \((s,t)\)-cut is defined as the sum of the capacities (or weights) in the cut. For unweighted (=equally weighted) graphs, this is simply the number of edges.

An \((s,t)\)-cut is minimum if it is of the smallest possible size.

References

JS Provan and DR Shier: A Paradigm for listing (s,t)-cuts in graphs, Algorithmica 15, 351–372, 1996.

Author

Gabor Csardi csardi.gabor@gmail.com

Examples


# A difficult graph, from the Provan-Shier paper
g <- graph_from_literal(
  s --+ a:b, a:b --+ t,
  a --+ 1:2:3:4:5, 1:2:3:4:5 --+ b
)
st_min_cuts(g, source = "s", target = "t")
#> $value
#> [1] 2
#> 
#> $cuts
#> $cuts[[1]]
#> + 2/14 edges from 87f32a6 (vertex names):
#> [1] s->a s->b
#> 
#> $cuts[[2]]
#> + 2/14 edges from 87f32a6 (vertex names):
#> [1] s->a b->t
#> 
#> $cuts[[3]]
#> + 2/14 edges from 87f32a6 (vertex names):
#> [1] a->t b->t
#> 
#> 
#> $partition1s
#> $partition1s[[1]]
#> + 1/9 vertex, named, from 87f32a6:
#> [1] s
#> 
#> $partition1s[[2]]
#> + 2/9 vertices, named, from 87f32a6:
#> [1] s b
#> 
#> $partition1s[[3]]
#> + 8/9 vertices, named, from 87f32a6:
#> [1] s b a 5 4 3 2 1
#> 
#>