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Journal Articles Theoretical Computer Science Year : 2024

## Constrained Flows in Networks

Jørgen Bang-Jensen
Stéphane Bessy
Lucas Picasarri-Arrieta

#### Abstract

The support of a flow $x$ in a network is the subdigraph induced by the arcs $ij$ for which $x_{ij}>0$. We discuss a number of results on flows in networks where we put certain restrictions on structure of the support of the flow. Many of these problems are NP-hard because they generalize linkage problems for digraphs. For example deciding whether a network ${\cal N}=(D,s,t,c)$ has a maximum flow $x$ such that the maximum out-degree of the support $D_x$ of $x$ is at most 2 is NP-complete as it contains the 2-linkage problem as a very special case. Another problem which is NP-complete for the same reason is that of deciding the maximum flow we can send from $s$ to $t$ along 2 paths (called a maximum 2-path-flow) in ${\cal N}$. Baier et al. (2005) gave a polynomial algorithm which finds a 2-path-flow $x$ whose value is at least $\frac{2}{3}$ of the value of a optimum 2-path-flow. This is best possible unless P=NP. They also obtained a $\frac{2}{p}$-approximation for the maximum value of a $p$-path-flow for every $p\geq 2$. In this paper we give an algorithm which gets within a factor $\frac{1}{H(p)}$ of the optimum solution, where $H(p)$ is the $p$'th harmonic number ($H(p) \sim \ln(p)$). This improves the approximation bound due to Baier et al. when $p\geq 5$. We show that in the case where the network is acyclic, we can find a maximum $p$-path-flow in polynomial time for every $p$. We determine the complexity of a number of related problems concerning the structure of flows. For the special case of acyclic digraphs, some of the results we obtain are in some sense best possible.

#### Domains

Combinatorics [math.CO]

### Dates and versions

hal-04379871 , version 1 (08-01-2024)

### Identifiers

• HAL Id : hal-04379871 , version 1
• ARXIV :
• DOI :

### Cite

Jørgen Bang-Jensen, Stéphane Bessy, Lucas Picasarri-Arrieta. Constrained Flows in Networks. Theoretical Computer Science, 2024, 1010, pp.114702. ⟨10.1016/j.tcs.2024.114702⟩. ⟨hal-04379871⟩

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